Additions are welcome! Please post in the comments if you’re missing any. While I realize other sources and shadow libraries exist, I want this list to be about supporting the sites, stores and authors that make an effort to supply legal, DRM-free alternatives.
Ebook Stores
Weightless Books (Sci-Fi magazines)
Publishers
Authors
Cory Doctorow (Sci-Fi)
Greg Egan (Sci-Fi)
Honor Raconteur (Fantasy/YA)
Juliet Marillier (Fantasy)
Brandon Sanderson on Bookshop.org (Sci-Fi, US-only)
Brandon Sanderson on eBooks.com (Sci-Fi)
Libraries
Project Gutenberg (American/General Public Domain, has many other countries and languages as well)
Project Gutenberg Canada (Canadian Public Domain)
Project Gutenberg Australia (Australian Public Domain)
Standard eBooks (Formatted Public Domain eBooks)
French eBooks
Ebooks libres et gratuits (French Public Domain)
French Bibiliothèque Nationale’s Gallica (French Public Domain)
Les classiques des sciences social, with a large selection of essays and academic papers
La bibliothèque numérique Romande (Swiss fiction)
Le Belial (Sci-Fi/Fantasy)
Dystopia Editions (Sci-Fi/Fantasy)
Nordic eBooks
Litteraturbanken (Swedish Public Domain)
Runeberg (Swedish Public Domain)
Nasjionalbiblioteket (Norwegian Public Domain)
The National Library of Finland (Finnish Public Domain)
Audiobooks
LibriVox (Narrated public domain eBooks)
Games
Music
Honorable mention, SomaFM: free and non-commercialized internet radio.
A note, if your native language is for example German, Swedish, Norwegian, Danish, Polish or similar, you can usually find DRM-free ebooks in your native language through your national stores. The ePubs are usually just watermarked. This might be applicable to other countries as well, even though I’m aware that some countries like Japan or South Korea have even stricter DRM schemes than the English-speaking world.
More on DefectiveByDesign, Libreture and ButtonDown
Brandon Sanderson offers many of his ebooks on bookshop.org DRM-free: https://bookshop.org/beta-search?keywords=Brandon+Sanderson
Thanks, unfortunately US only but Sanderson is published by TOR who pretty much releases all of their books DRM-free.