my favorite in the moments:
www.mindmeister.com:
allows collaborative brainstorming using mind maps. excellent implementation, really helpful for making up your mind, once you got used to it. (free version available)
simultaneous real time editing of texts. start immediately without registering. got a unique URL directly accessible (free version available).
www.mixedink.com
another collaborative text editor, but this one has a special feature: the site takes a "democratic" decision, based on voting of contributors within a certain deadline.
also interesting: you are supposed to pick text parts of other versions of other users, the system tracks which text part originally came from which contributor to grant partial authorship in the final version.
