Additions are welcome! Please post in the comments if you’re missing any. While I realize 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 ebook alternatives.
Stores
Weightless Books (Sci-Fi magazines)
Publishers
Authors
Cory Doctorow (Sci-Fi)
Greg Egan (Sci-Fi)
Honor Raconteur (Fantasy/YA)
Juliet Marillier (Fantasy)
Libraries
More on DefectiveByDesign
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.