These are the ones I've used in the past:
http://manybooks.net/
http://www.baenebooks.com/c-1-free-library.aspx
http://www.smashwords.com/ - Free books can be downloaded directly, no "membership" required.
The thing I like about some of the "non-Amazon" book sites is that you can find independent authors and sometimes there are some real gems in there.