Joyce Saricks calls a genre "any sizable group of fiction authors and/or specific titles that have similar characteristics."  There are about as many genres as there are readers with the imagination to dream up new categories. 

The categories highlighted on this page include:

Adventure Fiction
African-American Authors
Asian-American and Pacific Islander Authors
Animal Stories
Cult Fiction
Fantasy
First Chapters Available Online
Gay, Lesbian, Bisexual, Transgender, & Queer Perspectives
Geographic Locations
Graphic Novels & Other Comix
Historical Fiction
Horror
Humor & Satire
Indigenous Authors
International Authors
Latinx Authors
Made into Movies (or TV Shows)
Memoirs & Biographies
Mysteries & Thrillers
Poetry
Romance
Science Fiction 
Short Stories & Essays
Travel & Adventure
True Crime
War
Westerns
Women Writers
Young Adult

50 Must-Read Adventure Novels - from Book Riot
Adventure and Military Fiction - a collection of links to booklists put together by public librarians
Anchors Aweigh - an illustrated and annotated list of classic naval adventure fiction from the Lincoln City Library
LibraryThing Subject List: Adventure Fiction

African American Literature Book Club - book reviews, bestseller lists, author pages, and more.
Black Resilience - a list from the Cincinnati Public Library
African-American History Month: Non-Fiction - A broad and deep list from the Los Angeles Public Library, updated each year.
Great Books for Teens by African-American Authors - Some YA classics from the Burlington County Library System in New Jersey.

32 Essential Asian-American Writers - from Buzzfeed
APALA Literature Awards - annual awards that "honor and recognize individual work about Asian/Pacific Americans and their heritage, based on literary and artistic merit" across a variety of categories
Asian American Writers, Their Books, and the Books They Love - a beautifully intertwined suite of recommendations from the American Writers Museum
Beloved Asian American Literature - a guest post from Karissa Chen on the NYPL blog that features a splendid range of genres and styles.
Marginalia - posts about culture (including lots of book recommendations!) from the Asian American Writers' Workshop

Endangered Species - A list of nonfiction books from the Seattle Public Library
Fiction Featuring Animals and Pets - a list of public library booklists: everything from "Animal Whisperers" to "Snake Fiction".
Mysteries Featuring Animals - a second list of public library booklists, this one focusing solely on mysteries.
If You Like Marley and Me - Lincoln City Library's list of "sentimental, humor-filled" animal memoirs.

50 Coolest Books Ever - an annotated list from The Shortlist
Classic Cult Fiction - this Art and Popular Culture wiki article includes a list taken from Thomas Reed Whissen's book of the same name, with links to individual articles covering most of the authors and titles listed.
Cult Classics - from the Seattle Public Library

Diverse Futures - a labor of love promoting Black speculative fiction (fantasy, horror, science fiction, etc.)  Incredibly comprehensive lists, and also some links to other resources.  Reviews from the main creator of the site can be found at the associated goodreads page.
Top 100 Fantasy Books of All Time - chosen by the writers of Fantasy Book Review.
Mythic and Fairytale Fiction Reading Lists - from the now-defunct (but still full of wonderful stuff for fantasy fans) Journal of Mythic Arts.
The Recommended Fantasy Author List - an incredibly completist list of fantasy novels recommended by somebody at some point in time (often a member of the rec.sf.arts.eddings newsgroup back in the early days of the internet). 
tor.com - fantasy/scifi imprint Tor Books has created a vibrant community site focusing on all things SFF-related. Art, book reviews, chapter-at-a-time rereads of fantasy classics, blog posts from popular writers (including ones who don't publish with Tor), and more. 
Urban Contemporary Fantasy - suggestions from the Fantasy Book Review
World Fantasy Awards

BookBrowse - this top-notch review database offers excerpts for almost all the books it contains (enhanced for Tutt Library patrons)
CNN - features mainly, but not exclusively, adrenaline-charged thrillers and spy novels.
ReadersRead.com - this site features an eclectic mix of excerpts, sorted by genre.
USA Today - a selection of popular fiction and nonfiction, reaching back about 10 years.

I'm Here, I'm Queer, What the Hell Do I Read? - wide-ranging book reviews (helpfully arranged by subgenre in the left sidebar), interviews, news, other blogs of interest, and more.
LAMBDA Literary - home of the LAMBDA Awards for GLBTQ fiction, this site also hosts a book blog, a review site, and an online book club.
LGBTQ+ Literature - features a long list of recommended titles, as well as other resources
Over the Rainbow - an annotated list of recommended books from the GLBT roundtable of the American Library Association.

Bookmap - this site uses the Mappit! backbone to link books as specifically as possible to the places they are set in.
Goodreads Places Guide - this enormous list compiles all the place name tags used by Goodreads members. There are thousands of locations listed, with at least a few books for each location.
TripFiction - searchable, sortable annotated database of novels by location.
World of Books - "annotated surveys of noteworthy books from around the globe" compiled by the Library of Congress

Best of No Flying, No Tights - staff recommendations from the most extensive graphic novel review site on the web
FA: The ComicZine - commentary, news, and reviews
Great Graphic Novels for Teens - a list from the Young Adult Library Services Association
TIME Magazine’s All-TIME Graphic Novels

Historical Fiction - list of all historical fiction reviewed/excerpted/etc by BookBrowse
HistoricalNovels.Info - searchable in-depth book reviews, a news blog, web links, and even a serialized novel!
Reading the Past - Sarah Johnson is a librarian who has published books about helping readers find good historical novels. Her (indexed) review blog also includes author interviews, upcoming books, and a feature on tacky cover art.

Bram Stoker Awards  - winners and nominees of the prime US horror award
Horror Reading List - a short list of classics recommended by the Horror Writers Association
Monster Librarian - horror-loving librarians (and friends) collaborate on this site: reviews, author interviews, and other reading resources
The Seers' Table - monthly recommendation newsletter from the Horror Writers Assocation, highlighting works by horror writers who aren't straight white cis guys.
Shapeshifter Emporium - browseable, illustrated lists of fiction, nonfiction, movies, etc. about werewolves and other shapeshifters

Archives for the "Humor" Category - at Blogging for A Good Book.  (They also have a "dark humor" category.)
Humor and Satire - book recommendations, reviews, and excerpts from BookBrowse
Leacock Medal - awarded annually to the funniest Canadian book published in the last year.
The Ultimate Comedy Library - Vulture's thoroughly annotated (and categorized) list of "90 books every comedy fan should read"

22 Books by Indigenous Authors - recommended by the Chicago Review of Books
First Nations Knowledge Center: Books - a comprehensive recommended reading list by the First Nations Development Institute; this nonprofit is led by an all-Indigenous board of directors.  The site offers many other educational resources such as videos and recipes. Their #NativeReads campaign is also excellent.
Read Indigenous - a yearly list of recommendations from Toronto Public Library, selected in cooperation with the TPL Indigenous Advisory Council.

African Book Addict - "Book reviews and recommendations of books from Africa and the Diaspora"
Reading Globally (LibraryThing) - fiction recommendations and book discussions, organized around regional themes (for example, "20th and 21st Century Central and Eastern Europe," or "China and Its Neighboring Countries").
Words without Borders - "the online magazine for international literature": news, reviews, award winners, fiction, essays, and award announcements.
World of Books - "annotated surveys of noteworthy books from around the globe" compiled by the Library of Congress

America Reads Spanish - book reviews and news from a website that strives to promote Spanish-language reading in the US
Latin American Literature - book reviews from mostlyfiction.com
Libros, Pelicúlas, Música - Spanish-language reader's advisory from Denver Public Library
Pura Belpre Award Winners - given to Latino/a authors and illustrators of children's books about the Latino experience

The 45 Best Book-to-Film Adaptations Ever - from Shortlist
Based on the Book - Mid-Continent Public Library's highly-organized list of more than 1250 books that have been adapted to film
Books into Film - these booklists were compiled by Christchurch City Libraries, in New Zealand.
Books on Screen - a blog from Book Reporter about upcoming films based on books; updates monthly

Citizen Reader: Memoir - Sarah Statz Cords applies her usual humor and verve to a lot of memoirs, and she doesn't hesitate to dish out lavish praise or harsh words. You may find yourself enjoying her reviews of books you have read as much as her suggestions for those you haven't!
PEN/Ackerley Prize - given to a British "literary autobiography of excellence" every year. Their website includes nominees as well as winners: a rich source for finding elegant writing about people's own lives.
Pulitzer Prizes for Biography or Autobiography - this venerable American prize tends to reward political and historical biography. Recent winners tell the stories of George Washington, Cornelius Vanderbilt, and Louisa May Alcott.
A Reader's Place: Roz Recommends - Roz Reisner is a librarian who specializes in connecting readers with memoirs.  "Roz Recommends" collects several of her booklists, such as "Complications: Doctors Write About Their Experiences," and "Memoirs Set in Africa", but her whole site is worth exploring if you're a memoir fan.

The Big Thrill - this monthly webzine features reviews, interviews, giveaways, and more, under the aegis of the International Thriller Writers.
Eurocrime - a review and news blog focusing on British and European crime fiction, tv shows, and movies.  Great for fans of Stieg Larsson and BBC Mysteries, this blog also explores more obscure corners of international crime.
Reviewing the Evidence - news blog and mystery review repository.  This site doesn't have many bells and whistles, but it does have thousands of thoughtful, independent reviews.
Stop, You’re Killing Me! - voracious and discriminating mystery readers love this site. SYKM not only reviews mystery novels, it also features indexes that let you hone in on your preferred style of mystery, categorizing titles by location, historical time period, protagonist's day job, or protagonist's diversity of experience.  There are also readalike lists and award-winners.

Best Poetry Best of Lists - this list is compiled by Poetry Foundation staffers. If you search their site, you can peruse lists from earlier and later years.
Guardian Poetry Page - the Guardian is a UK newspaper known for its extensive and thoughtful book coverage. You can browse their poetry-specific articles at this link.
Poetry Foundation Collections - this page is full of many amazing collections of free poems by critically respected poets, organized thematically around topics as widespread as Islamic Faith, Asian American Voices, and Baseball Poems.
Publishers Weekly Poetry Reviews - this tumblr links to every poetry review in Publishers Weekly soon after they are published. 
Small Press Poetry Bestsellers - this site publishes monthly lists of best-selling small press poetry books.

All About Romance - we're linking to this site's workhorse of a power search, which lets you search not just authors and titles, but also type of story, location, time period, sensuality level, and lots more.  However, the site as a whole has tons of other swoon-worthy features: everything from message boards to a Top 100 Romances reader poll to the Annual Purple Prose Parody Contest.
The Emma Awards - this award celebrates the very best "published romance novels, novellas, and women’s fiction featuring diverse and multicultural protagonists (such as black, African-American, Latino American, Hispanic Americans, biracial, etc.)".
Love Stories - BookBrowse's list of literary romances includes not just recommendations, but also excerpts, readalike suggestions, and in-depth reviews.
Smart Bitches, Trashy Books - the name of this site has probably already tipped you off that it's not well-suited for readers with delicate sensibilities.  The brash and vulgar, on the other hand, will find lots to adore about SBTB, including blunt-but-affectionate reviews, snappy podcasts, and (really!) thoughtful commentary about all things romance.

Center for the Study of Science Fiction - an intensely academic site hosted by the University of Kansas under the auspices of noted science fiction author/scholar James Gunn.  Lots of information throughout, although a more casual enthusiast might rather skip straight to the Resources section....
Diverse Futures - a labor of love promoting Black speculative fiction (fantasy, horror, science fiction, etc.)  Incredibly comprehensive lists, and also some links to other resources.  Reviews from the main creator of the site can be found at the associated goodreads page.
The Internet Speculative Fiction Database - like the IMDB, only for SF novels.  Great for finding out more about a particular author or series!
io9 - eclectic, active blogging community
Locus - this venerable SF magazine has the most active web presence of them all
SF Site - focusing mostly on reviews, etc
Strange Horizons - new short stories, reviews, and critical essays every Monday.  Strange Horizons has been around for more than a decade; it's an excellent place to find great writers before they make it big.
tor.com- Free stories and art, thoughtful reviews and rereadings, and conversations about speculative fiction both profound and frivolous. This site is published by a genre publisher (TOR/Forge), but its content has no particular editorial bias.
Uchronia - alternate history (aka alternative history, counterfactuals, "what if" books, etc.) - a massive, searchable, annotated bibliography for this subgenre

Best/Favorite Books of Essays - this Goodreads list offers up essay collections from a wide variety of genres and time periods, ranked by popular vote
Sample Tutt's Collections - Tutt has a long run of the Best American Short Stories and Best American Essays, as well as many volumes of the Pushcart Prize series. Anthologies can be very useful in the hunt to find new authors to love!
Short Stories and Essays - a long list from BookBrowse with lots of short-form reading, both fiction and nonfiction
Short Stories in the Spotlight - a tasty mix of hot new titles and canonical short-story writers
Short Stories We Love - 19 short story collections recommended by Oprah's Book Club

Books for Armchair Travel and Adventure - Famous librarian Nancy Pearl recommends some of her favorite transporting reads
Essential Books for the Well-Read Explorer - Outside magazine's top 25 non-fiction adventure travel books.
Travel and Adventure - BookBrowse recommends more than 90 books in this category, with reviews and excerpts for each title.

10 Best True Crime Books of the Past Decade - covers the twenty-teens, from the excellent mystery and true crime site CrimeReads.
Book Browse True Crime - this list adopts an inclusive view of the genre, including not just the usual suspects, but also books about Nazi Germany, Stalin, and Guantanamo. Like all Book Browse lists, it links to reviews and excerpts of each book.
True Crime: Books, Podcasts, and Documentaries - YALSA's suggestions for young adults; great for not-so-young adults too!

Books About Veterans and Their Families - a list from the Oakland Public Library, organized into categories
Guide to Civil War Novels - A serious and well-organized academic site that is also useful for fans
A Large Handful of Novels About World War II - personal recommendations from the webmaster of Flashlight Worthy

Best of the West - personal fiction and non-fiction recommendations from western writers and thinkers, from the pages of the High Country News
Official Louis L’Amour Website - Mr. L'Amour is the universally acknowledged Western grand master.
Readwest - Readwest is a non-profit foundation "dedicated to increasing the awareness and diversity of western literature"
Spur Award Winners from the Western Writers of America, Inc. -  awards for all western genres: fiction, nonfiction, plays, kids' books, songs, and more!
Willa Awards from Women Writing the West - these awards honor "women's or girls' stories set in the West"

17 Essays by Female Writers That Everyone Should Read - most of these essays are freely available online - a great way to discover an author you haven't read before!
Feminista!‘s 100 Great 20th Century Works of Fiction by Women (from LibraryThing) - compiled in response to Modern Library's 100 Best Novels list
Top 10 Women Authors of My Misspent Literary Youth - part of the New York Public Library's series on celebrating Women's History Month, this enticing and deeply personal essay recommends some classic works by not-always-canonical authors
Voices from the Gaps (VG) - this site takes an academic approach to the works of women writers of color. If you're willing to dig a bit, there are a lot of interesting interviews, reviews, and more...

Best Books for Young Adults from YALSA - Each year, YA librarians and other selectors pull together all kinds of best book lists. Includes adult and young adult titles that are good for young adults (and the rest of us!) to read.
Reading Rants! Out of the Ordinary Teen Booklists! - check out the sidebar for the lists referenced in the title of this YA librarians' review blog. She also does a great job linking to other YA recommendation blogs, and to popular young adult authors.
Rich in Color - reviews of YA books, focused on new releases, with protagonists and/or authors who are Black, Indigenous, or otherwise people of color / part of the global majority. 
TIME's 100 Best YA Books of All Time