In Ars Technica, last October, I saw a story about a professor that assigned her students the task of writing a Wikipedia entry. I went one better, I think...Natalie Glance and I were teaching a seminar in "Analysis of Social Media" this fall, and I assigned the class the task of building a wiki on the subject. During the class I limited wiki access to class members, but it's now open to everyone to read or edit.
It was an interesting experiment. It was only a little extra work in grading and coordinating, but it was worth it for the irony factor alone. The students were mostly positive about it.
The principle content of the wiki is a bunch of paper summaries, not unlike what students would have turned in in a class, but some students came up with some other ideas for contributions. The main change I'd make if I did this again would be switching wiki providers. (Cheapskate that I am, I used a free wiki farm called scribblewiki. They were very helpful early on, and even upgraded me to a "paid" account for free, but took a long time with some other requests later - and never did add support for latex math.) It's not obvious what's the best way to use a wiki in teaching course for the (n+1)th time, but it's a fun twist for the first time you teach a seminar.