Check out Amazon own free books. The free books are in the right hand column and change frequently. Be sure to check before clicking every once in a blue moon some pay to play books end up in the right hand column, but not often:
http://www.amazon.com/gp/bestsellers...ag=dealnewscom
Be sure to bookmark the site so that it is handy in the future, and check back often (once a day)
You might also find some interest in:
http://books.google.com/