Wikipedia
Wikipedia is a great concept as it builds on the natural way in which knowledge is developed within society. The success of the original idea has resulted in several specialist sites using the same principles. Joseph M Reagle Jr has written an interesting study outlining the coperative elements of Wikipedia - its a little heavy going in places, but don't be put of by that as it is illuminating. Thanks to the excellent Wikibilio for the reference.
Meanwhile the Q & A website features a lengthy interview with Wikipedia founder Jimmy Wales

Wikipedia:
http://en.wikipedia.org/wiki/Main_Page

Reagle: A Case of Mutual Aid: Wikipedia, Politeness, and Perspective Taking
http://reagle.org/joseph/2004/agree/wikip-agree.html

Jimmy Wales interview (Q & A):
http://www.q-and-a.org/Program/?ProgramID=1042

Wikimedia Foundation:
http://www.wikimedia.org/

Wikibiblio
http://blog.lib.umn.edu/kenne329/wikibiblio/