Librivox - volunteer, open source project provides free audiobooks from the public domain (mp3, ogg)
Google Books - classic public domain books scanned to PDF format. Search the full text of books.
Freebook Sifter - lists 35,000 free eBooks on Amazon.com by category.
Bartleby reference books - great books online
Our Roots - Canada's local history
University of Pennsylvania digital library has over 20,000 free books online:
"Online Literature Library" - collection of classics
University of Texas (Austin) Online Library - many books are limited to UT Austin students
Math.Gatech.Edu - math textbooks from Georgia Institute of Technology
Arxiv.Org - Articles on astrophysics, relativity, quantum cosmology, high energy physics from Cornell University
The Creative Commons - novels, photos, media, music
NetGuide Best of the Web Reviews - searchable digital library
National Academy Press -searchable, 3000 publications (free)
The Invisible Web - libraries, databases
World Wide Web Virtual Library - maintained by volunteers
PDQ Library is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.