I want to repeat wayspooled's recommendations. Go check out the Baen free library.
www.baen.com/library
It has both fantasy and science fiction. Baen is a leading publisher in the field that gives away many books as a move to gain publicity. In my opinion, they are the top source for modern era free books.
You will also find many free books on
http://www.gutenberg.org/wiki/Main_Page and here on Mobileread. Most of these are 'classics' that were written before 1923. Quite a few folks really enjoy them, but for me they usually aren't to my taste. One exception would be the novels by Andre Norton. Type "Norton, Andre" into the Mobileread search box and you should find a couple books by her.