Wetpaint's Website allows one a easy way to create free wiki websites. They invite you to "please touch: create or join a wiki. . . build a community around whatever you love" You have probably known of Wikipedia - it's the online encyclopedia that is literally created by a community of users. Wikipedia has grown rapidly into one of the largest reference sources, attracting at nearly a billion users in 2008. There are some other wiki-generators, but I really like Wetpaint for its look and easy of use. You can watch several videos to find out more here.
Wikis are often used by groups to create a content-based Website with a restricted community of users – in other words, a private wiki. However, like other Web 2.0 applications, Wetpaint can take to existing wikis that may be something to join or at least to build from. In addition to being a tool to create a social website that anyone can easily edit and contribute to, there are many existing Wikis that can be accessed. For example, you can see the Efficient MD Wiki or WikiBirdFlu.