Definitely one to watch -- go create a free wiki for yourself and check it out
Short version: Wetpaint might be one to watch.
Long version: TechCrunch's Michael Arrington has alerted us to a dark horse candidate in the race to dominate the land of wikis. It's Wetpaint, a Seattle-based service we haven't heard a whole lot from lately. The reason, Arrington says, is that it's positioning itself to be a player in niche social networks, not just mini-Wikipedias.