Just found this great Web 2.0 app, thanks to Mashable's 60+ Collaborative Tools collection.
Thinkature is different from wikis in that it allows for real-time collaboration. Group members can edit the same work space at the same time, which makes it great for planning. The site allows instant messaging AND voice chat. Text is type in color coded boxes. A rudimentary drawing tool is also available.
You really couldn't complete a project or a paper here, but it's a great place to brainstorm ideas.
Other interesting collaborative tools:
Writeboard: Think Google Docs meets wiki. Online document collaboration that saves all versions and allows you to revert or compare.
Novlet: Novlet is a web application designed to support collaborative writing of non-linear stories in any language. With Novlet you will be able to read stories written by other users, create your own, and choose the plot you like most from several alternatives.
Kalabo: Allows musicians to collaborate, download free original music, or remix.