Open Books is a compilation non DRM e-books from various sources linked to enable readers to browse and download them.
So far the e-books are generally coming in via Smashwords, Closed Circle, BeWrite Books, and Carina Press. The cost for each e-book is fairly small, generally under $5, sometimes much less than that. The user is asked to abide by an "honesty" system that does not encourage piracy.
The e-books are generally available in .mobi (Kindle), epub and pdf.
One of the great pointers to e-books on the Project Gutenberg site is the Magic Catalog.
Amazon has a few DRM-free e-books. Look for "Simultaneous Device Usage" under "Product Details" and if it is set to "Unlimited" then the book is DRM-free.