ipl2
http://www.ipl2.org

WWW Virtual Library
http://vlib.org

Multnomah County Library Homework Center
http://www.multcolib.org/homework

Dewey Browse
"Web Sites Classified by the Dewey Decimal Classification System for Grades K-12"
 http://www.deweybrowse.org

The Internet Archive
a digital library of Internet sites and other cultural artifacts in digital form”
http://www.archive.org/

Europeana
Gateway to books, paintings, photos, films, as well as all kinds of museum and archive items from European institutions.
http://europeana.eu/

World Digital Library
http://www.wdl.org
 
American Memory - Library of Congress
http://memory.loc.gov

Hathi Trust Digital Library
http://www.hathitrust.org/

Google Books
A lot of the content in Google Books isn't free or available as full-text.  So, use the advanced search and check "full view only" to get free, full-text material.
http://books.google.com/advanced_book_search

The following sites/gateways/collections are excellent resources for students in college-level courses:
Voice of the Shuttle
http://vos.ucsb.edu/

INFOMINE
http://infomine.ucr.edu

Rutgers University Libraries Subject Research Guides
http://libguides.rutgers.edu/

Fordham University Research Guides
http://www.library.fordham.edu/researchguides/guides.html

DOAJ – Directory of Open Access Journals
Free scholary journals from around the world.
http://www.doaj.org/