http://www.mobilism.me for books and audiobooks (my #1 favorite, requires free account to download, it has around 90% of what I look for, including novels, a great collection of cookbooks, magazines, etc).
ebooks channel on IRCHighway for ebooks (my 2nd favorite, has pretty much anything mobilism doesn’t, but requires a free IRC client… here is a guide on how you can access it)
http://www.libgen.fun for technical manuals, textbooks, and magazines (my 3rd favorite, also has mirrors with different suffixes like libgen.rs, .is, .lc, .gs, .nl, etc)
http://libgen.rs/fiction/ is the ePub fiction section of libgen (thanks to u/deadnamingmissdaisy for pointing out that libgen has various sections depending on what you’re looking for)
http://www.myanonamouse.net (relatively complicated to gain access to in comparison to the other sites, but generally higher quality materials - it’s a private tracker and requires an application and a 1-on-1 live chat interview with study questions)
What are yours???
https://3lib.net (thanks u/h4llobr3; no account needed. Possible a mirror of z-lib.org?)
http://sci-hub.tw/ (for technical papers, thanks to u/julianvgs)
https://tokybook.com (thanks to u/callmeultimate, it appears to have popular audiobooks set up for mobile listening via the app)
https://oceanofpdf.com (thanks to u/thenebulawolf)
http://www.freefullpdf.com (thanks to u/classicdannie, mostly legal, for scientific articles and journals)
https://www.freetechbooks.com/ (thanks to u/psuedopoder, legal technical publications)
open directories of Calibre lists (thanks to u/PM_ME_TO_PLAY_A_GAME, various novels and ebooks)
https://www.ebookelo.com (thanks to u/gexgekko, mainly ebooks in spanish/other Iberian Peninsula-located languages, with some English books)
https://ebook-hunter.org/Books/ (thanks to u/pelumo_64, ebooks)
http://www.pdfget.com (thanks to u/pelumo_64, various books and periodicals)
https://sanet.st/full/ (thanks to u/pelumo_64, various books, textbooks, periodicals)
http://www.getcomics.info (thanks to u/antlereye, comic books with an option to either read online or download)
https://flibusta.site (thanks to u/decumos, Russian ebooks)
https://the-eye.eu/public/Books/ (thanks to u/WhiteMilk_, eclectic compilation of various stuff, including some Calibre lists)
https://trantor.is (thanks to u/fierze16, Imperial Library of Trantor, darkweb ebook resource - slow because the clearweb address interfaces with the darkweb site)
https://galaxyaudiobook.com (thanks to u/MyriadThings, audiobooks)
https://goldenaudiobooks.com (thanks to u/DeliciousFeet, audiobooks)
https://mywarez.org/ (thanks to u/gcar1966, has an ebook section with 40k+ titles of very random stuff, and an audiobook section with 1500+ titles. Requires registration.)
Also, here are two honorable mentions. Legal, but worth a look.
https://archive.org/details/southerncookbook00lustrich/page/34/mode/2up (this one is legal. It’s a collection of around 11,500+ scanned cookbooks from the 1700s to today. Most you can view without an account, but the newer/more popular ones require a free account to “check out” the book for an hour… it’s put together by UCLA and some other universities. Just be warned that some of the older southern cookbooks have references to “n words”, like at the bottom left of this page.. try to remember the time in society in which the books were written when browsing).
http://www.gutenberg.org (thanks to u/katzenpippi, mainly books that are out of copyright. Totally legal website).
On a side note, I use “Calibre” to manage all of my ebooks. It’s free, and can convert from one file type to another, plus it allows sideloading and emailing books to your device.