I've never paid as much attention to wikis as I have to blogs. In part that's because I'm always looking for a way to actually use the technology in my own teaching. Because of the totally asynchronous nature of AU courses, I've had difficulty seeing the applicability. I've also noticed that when it comes to using wikis, most of us prefer to write comments than actually change or add to something someone else has written.
Nevertheless, as part of the Becoming a Webhead course, I decided I had to develop a wiki. I gave it a title as though I were going to use it with one of my classes. I put two exercises there and linked to the wiki tutorial that comes with pbwiki. One of the exercises is designed to try to show students the way to actually use a wiki--they are required to add information.
There are also some other recommended wiki hosting sites: