The new Open Library demo site launched by Internet Archive comes very well recommended. Cory Doctorow blogged about it on BoingBoing, noting the word came from Rick Prelinger at blackoystercatcher - and pointing out the the Open Library is led by Aaron Schwartz from the idea of Brewster Kahle. The Open Library demo site says:

Imagine a library that collected all the world’s information about all the world’s books and made it available for everyone to view and update. We’re building that library. Search it: