I'm new to wikis so for those old hands out there this will be nothing new. I have experimented with pb wiki today. This looks like it could be a good tool for classes to use to develop a shared understanding of a topic. All the class members can participate in building knowledge by adding to the content.
Wikipedia is the classic example of this of course but with something like pbwiki you can create your own class space and only allow those who you want to access to edit it.