Diesel is a major ebook supplier and always worth checking out.
To get free ebooks go to their Free Ebook page where they have a selection of current, free ebooks.
For low-priced books, check out their home page where, on the left, they have an Under $5 link to general fiction, romance, fantasy, sci-fi and mystery.
Diesel books is really nice for finding great deals on e-books. You can also get free ebooks from sites like manybooks.net, ebookjunkie.com, free-ebooks.net. There are many more sites like this. I wish there was time to list them all.