Group of 25 Major Research Libraries Offers Full-Text Search to 4.6 Million Digitized Volumes
(Nov. 20, 2009 Chicago Tribune)
Hathi Trust is a scanned-book repository similar to Google Books, but in this case it's run by libraries.
With this site plus Project Gutenberg and Google Books, I have access to enough nerdly, obscure works to keep me happy to the end of my bookwormish life.
The interface is easy to use. The options for narrowing a search are far more numerous than those on the Gutenberg, and give more relevant and consistent results than Google. Books are displayed one page at a time, and, on a very slow connection, take a mildly annoying but not unacceptable time to load .
Unlike Google (sometimes, albeit only in annoying PDF format) and Gutenberg (always, in robust and convenient plain text and HTML), there doesn't seem to ever be any option to quickly download an entire book for offline reading. There is, however (with the inconvenience of setting up an account and logging in every time), a way to set up a "personal collection" on the site, and bookmark pages. Gutenberg is still the best site for collecting your own supply of portable books on a flash drive.