Inspired by Google Maps and Wikipedia, Wikimapia is a map/wiki mashup where one can add a wiki entry about any place in the world. It works with Google Earth, and GPS devices, and requires the community to vote on the addition of places, which is a good way to ensure a critical mass of people contributes to the entries on places.
The Startup School Wiki was created to organize all the information generated at the 2006 Startup School held at Stanford University. This is a good example of how a wiki can be used for backchannel information gathering during a conference session or lecture. Imagine how deeply engaged students could be if you did this for your class!