Project Gutenberg http://www.gutenberg.org/
Google eBookstore http://books.google.com/ebooks?uid=117522004192189783614&as_coll=1040
Science Fiction e-books http://www.e-booksdirectory.com/listing.php?category=59
20 Best websites for ebooks http://www.hongkiat.com/blog/20-best-websites-to-download-free-e-books/
Wowio http://www.wowio.com/