A few I use include:
- ebooks.com, which offers epubs and pdfs of most titles available on Kindle, usually at a somewhat higher price than the Kindle format.
- audiobookstore.com, which has a lot of audiobooks in DRM-free formats, usually both .mp3 and .m4b. Excellent reading quality, a good option if you care about archiving your audiobooks. With their monthly membership plan the prices are manageable.
What about other people?