I have, in the past, used class assignments involving Wikipedia.  These usually involved students identifying weaknesses in entries dealing with the subject of the class, making the changes in Wikipedia to remedy these weaknesses, and then tracking the entry to see if anybody else modifies or deletes their changes.  I always found these assignments to be useful and illuminating, especially watching students change their stance toward Wikipedia from one of deference to incredulity.  Over the last couple of years it has, for various unimportant reasons, fallen off my list of preferred classroom techniques, but now seeing this post on the Profhacker blog, with links to a wealth of resources, is inspiring me to integrate it again into one of my classes.  Keep your eyes on the “Adam” and “Eve” entries – you never know what might happen by the end of the semester.