This is a small collection of library databases that will link you to a wide variety of useful references.

Librarians' Internet Index 

http://www.lii.org

The Librarians' Internet Index is a great collection of refernce sites that cover a wide range of diciplines.  This is an excellent jumping off point for reference projects.

Library of Congress 

http://www.loc.gov/rr/askalib/virtualref.html

The Library of Congress maintains this "virtual reference desk".  It links to useful tools on a very wide range of topics.

Internet Public Library 

http://www.ipl.org

The Internet Public Library is a great reference resource.  One particularly useful feature of this site is its links to a large number of national and international newspapers.