Baen Books: There are about 120 titles available in the science fiction and fantasy genre for PC or Kindle.
Book Lending: Registration is required. Supports Kindles and you may lend or borrow a book between other users.
Digital Book Index: This site supports iPad, Kindle, and Nook readers. You may search by title, author, title and author, or title keyword among its 165,000 digital collection.
E-Discover the Classics: Looking for a great classic but don't want to search hours to find them? Kindles, iPads, Nooks and many other readers are supported.
Inkmesh: Free and easy to use, Inkmesh has sort by device, free or for cost e-books. It hosts about 3,500 e-books in fiction and nonfiction.
Open Library: Need children's books? Open Library includes title from popular Golden Books. Formats include but not limited to PDF and EPUB. Membership required.
Don't forget to check out your local library for their digital collection.