Another good source of free ebooks: Usenet
There are several ebook groups. I've been trolling them for about a week now, the two most popular seem to be:
alt.binaries.e-book
alt.binaries.e-book.flood
There is a FAQ at:
http://ebook.23ae.com
Pros: Huge selection of titles, many of them I haven't seen anywhere else.
Cons: Many are pirated (although many are public domain as well), and not very convenient if you are looking for a specific title.