General Reference

 

Gale Reference Library
Bartleby - Collection of Reference, Verse, Fiction, and Nonfiction including: Quotations, Thesauri, Dictionaries, Encyclopedias and more. It's all paid for by advertising.
Infoplease - Another advertising driven reference site with Almanacs, Thesaurus, Dictionary, Atlas, and Encyclopedia.
Internet Public Library - The IPL is a public service organization and learning/teaching environment at the University of Michigan School of Information. Basically it is a trustworthy index of countless useful sites.
Wikipedia - This is a fantastic publicly edited resource. It is a great starting point, especially for pop culture, but don't forget to check references. Anybody (including you) can change this information. It is especially hard to trust when it is discussing controversial issues or political figures. People will always put their own spin on history! Michael Scott from 'The Office' gives a very thorough explanation.

Medical Reference

Pubmed - Pubmed was developed by the National Center for Biotechnology Information at the National Library of Medicine located at the United States National Institute of Health. It grants free access to thousands of medical journals and full text articles. This is a great place to start with any biological or medical research.
MedLine Plus - A repository of medical information brought to you by the U.S. Government
Health Reference Center Academic
Health and Wellness Center

Misc.

 

Google Goodies:

Google Books - A great place for primary sources and moldy oldies.
Indexed Biography Resources
Indexed Book Review Resources