Bibliomania contains public domain materials as well, but includes illustrations and fonts that are far easier on the eyes than Gutenberg's.
Classic Bookshelf is another public domain book site that lets you customize colours and fonts for your reading comfort.
University of California Press has converted many of their out-of-print titles to electronic form. A lot of them are subscription-only, but around four hundred titles (with illustrations) are available to the public. ("A Buccaneer's Atlas" might be of special interest to pirate fiends ;-))
The Online Books Page at the University of Pennsylvania has a vast assortment of goodies.
The Baen Free Library (run by Baen Books, natch) contains various complete science fiction novels by Baen authors, all volunteers in this project. Yes, it's a publicity stunt and a good one ... gives readers a chance to really sample an author before committing to shelling out money for their book(s). The books are changed regularly, so there's always new titles to try out.