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Books

Standard eBooks Nicely formatted public domain books
Queer Liberation Library Libby library that's open to anyone in the USA
Japan Foundation Library Libby library that's open to anyone in the USA
Recommend Me a Book Shows you a brief preview of a book, only revealing which book it is when you click a button.
Unglue It
Digital Book Index
Libgen Wikipedia link used as working addresses change frequently
Anna's Archive Wikipedia link used as working addresses change frequently
HathiTrust Digital Library Old books and magazines
Project Gutenberg
Trans Reads
Directory of Open Access Books

Dictionaries & Related

Sort of loosely sorted from en-en to ja-en to ja-ja, but many resources have a variety of language offered which will probably muddle it

One Look Searches hundreds of E-E dictionaries at once. The search function is insanely powerful making this site really useful for word games
Stardict Dictionaries A bit difficult to navigate but covers a bunch of different languages and language pairs
Wordnik dictionary used by a lot of online word games
MDict Dictionaries for Goldendict Extremely large collection of basically every possible language pair including monolinguals
Tip of My Tongue Lets you search for words in unconventional ways
Green's Dictionary of Slang Very high quality dictionary of English slang, including when the term originated
Rhyme Zone Find you words and phrases that rhyme with your query
Phrontistery
英辞郎 on the WEB J-E dictionary aimed at JP speakers
The Jaded Network J-E Sound effects. The site is disintegrating, but it's still usable
WWWJDIC The default J-E dictionary is the usual one you see in these resources, but there's several other dictionaries included
EPWing Dictionaries
MDX Dictionaries
Yomitan Dictionaries
Yomitan Dictionaries (Different Link)
Yomitan Dictionaries (Different Link)
JP Onomatopoeias Onomatopoeia dictionary that isn't actively falling apart at the seams
J-J Dicts Proton Drive Several J-J dictionaries
伏字検索○○○ Helps decipher censored words

Search Engines

Springer Nature Link Scientific research
Refseek Academic references
Marginalia Tries to surface non-commercial sites
Wiby Searches a pool of curated sites rather than crawling
Bielefeld Academic Search Engine Academic research
Worldcat Searches library catalogs. Probably USA only?
Impersonal Google search but with a fake personalization profile
Search Engines With Their Own Indexes Overview of what search engines exist for general purpose usage
Ao3 Search Operators Cheatsheet List of search operators that Ao3 has that you might not know about
Old'a Vista Searches old archived homepages from Geocities and Angelfire
Picryl Searches public domain images
Trace.moe Finds the anime a screenshot is from
Feedle Searches RSS feeds
VHSearch Neocities-only search engine. It's clearly at least partially powered by a custom google search, but it's cute and gave me useful and relevant results
Clew Another small site focused engine
Openverse Searches creative commons media

Textbooks

Openstax
IT eBooka
Free Programming Books
Directory of Open Access Books
You Don't Know JS
LibreTexts
Open Book Publishers
Practical Typography
Open Textbook Library

other

The Eye Large archive of primarily documents iirc. Currently down due to technical difficulties, but they've recovered in the past so I'm still linking them
Textfiles.com Internet history via archived text files
Remove Paywall
Old Usenet Replay of the first several years of Usenet. Starts a new cycle every year