Project Gutenberg is an ancient project that didn’t really get popular until eReaders took off. PG has compiled and hosts over 33,000 free eBooks…some were released free by their authors, but most have simply entered the public domain once their copyright has expired. Frugal Kindle users looking for content providers that didn’t start in A and end in mazon have been mining its free archives for years.
If you’re looking for more recent books but hate to spend money, The eBooks Bay will hook you up with torrents for your Kindle, Nook, or whatever else you’ve got. They’ve got a fair amount of recent fiction posted up in all manner of formats, but the site is dominated by technical manuals and computer books. Fiction Books Bay is a little more focused. There’s also this Ukrainian site that has plenty of popular english best-sellers right alongside other Ukraine classics.
ePubBud also has a bunch of books – mainly for children.
Calibre is an indispensable tool for eBook owners, supporting more than two dozen different eBooks as well as every file format (there are at least 18,) enabling you to read anything on anything. It’s got a great metadata editor too.
If you’re the type who likes to be read to, LibriVox might be interesting. Volunteers read books from the public domain and make their recordings available free of charge, kind of like a free books-on-tape thing. You get what you pay for with LibriVox though, because hearing Ethel from the Bronx read Shakespeare is not as engaging as you might think.