My favorites, sorted by frequency of purchase (or "purchase" in the case of freebies):
- Baen's WebScriptions
- Mobileread's free downloads (they're easily accessible here, so no link)
- Project Gutenberg (for stuff not yet converted here at MobileRead -- and I usually try to upload an automated conversion for the Sony Reader)
- Fictionwise (their DRM-free multi-format books only)
- The Baen Free Library. This one would be higher on the list, except that I've long ago downloaded everything they have available.)
Tor may make it on the list if they either keep doing the freebie thing or start making DRM-free ebooks available.
I think that hits every site I've purchased an ebook from (with the exception of the $1.45 I spent at the Sony store to cover the extra cost of the last book I bought using their "$50/reader credit" back when the PRS500 was new).