http://delicious.com/

Video - Overview

http://www.youtube.com/watch?v=x66lV7GOcNU

Edtags
Website - a social bookmarking site for educators

http://edtags.org/about.php

 
Diigo
Website -- a research and knowledge sharing tool

http://www.diigo.com/

 
What's a Wiki?
Video - Overview

http://www.youtube.com/watch?v=-dnL00TdmLY 

Wikipedia
Website -- an online encyclopedia

http://www.wikipedia.org/

Article -- A History Department Bans Citing Wikipedia as a Research Source

http://www.nytimes.com/2007/02/21/education/21wikipedia.html?_r=2&oref=slogin&oref=slogin