There are many free ebooks here in the Mobileread forums;
https://www.mobileread.com/forums/ebooks.php?forumid=126 is the LRF-format (Sony) area, and
https://www.mobileread.com/forums/ebooks.php?forumid=130 is for ePubs (which Sony now prefers).
The Mobileread wiki has lists of free ebook sites:
https://wiki.mobileread.com/wiki/Free_eBooks
Some of the largest are:
http://www.munseys.com
http://www.feedbooks.com
http://www.manybooks.net
Baen Books has a free library of science fiction ebooks at
http://www.baen.com/library/
Sci-fi author & copyfight activist Cory Doctorow releases all of his books as free ebooks at
http://craphound.com/
http://www.teleread.org/free-ebooks/ is Teleread's list of free ebook resources.
http://www.smashwords.com is a self-published ebook store, but it has a lot of free and "pay what you like" books.