Here is an example of a collaboratively built wiki. This was assigned to college students who had one week to build it. What topics do you teach that students could build collaborative wikis around?
This appears to be about the same as Glogster which I have heard a lot about on Twitter. I haven't had the time to delve into it too far (we just got our student machines yesterday!) but I do know that Glogster has edu accounts which have some advantages.