Currently Joe Kraus is showing off Jot. It's a cool wiki-ish app that does the standard wiki stuff but also allows you to e-mail pages around and rank users (giving them specific access based on that). Additionally it lets you use forms and include structured info on the page. One thing that looks really cool is the ability to integrate RSS feeds and google searches and all kinds of feeds like that.

This seems to be the main competitor for Socialtext who happens to make some really great wikis.