Much of the attraction that Web 2.0 applications is that these tools allow for very powerful and meaningful collaboration. The theory is that none of us is as smart (or dumb, depending) as all of us. Wikis are perfect example of how a community can be created and wisdom and knowledge shared without ever physically meeting the other members.
A wiki is a webpage that can be edited by any member of the community. Wikipedia is the most well-known example, and despite the reputation that it has, it is still the most powerful example out there of shared community knowledge. Numerous studies have been done, and despite what people think of it, it is as reliable as the venerable Encyclopedia Britannica. One of the reasons for that is that there are so many people reading and editing the pages at the same time. Another reason is that Wikipedia staff are constantly monitoring the pages and vetting the changes nearly as soon as they are made by a user. That's not to say that at the moment you access a page there may be an inaccuracy, but it's pretty fair to say that others are checking the facts and trying to keep a page current.
Having said all that, there are countless wikis out there that organizations, groups or even individuals have created for all sorts of reasons. One that I just found (thank you, PNAIS TechDirectors listserve!) is the ISTE Independent Schools wiki. The purpose of this wiki is to allow teachers and tech folks who work in Independent Schools to collaborate, share ideas and solve problems. Here, you'll find lesson plan ideas and support from others who teach in independent schools all over the country. What a great idea!