WikiWikiWeb is a series of virtual web pages that anyone can create, modify or delete as they see fit. Although discussions and pages tend to be computer-programming oriented (especially eXtreme programming) there is also a significant body of work on a wide variety of other topics as well.
I find that in addition to just raw creative entertainment value, Wiki is also useful as a resource for relatively rapid-response relatively high-expertise level answers to computation-related questions.
Wiki is the brainchild of Ward Cunningham and there are now several other independent versions and derivatives dedicated to a number of differing themes.