Early in the 2009 school year, a post was done about Webspiration. This is still a great collaborative visual organizer! For some, however, it may have too many choices and unfortunately, in the Fall of 2010 the public beta will be over and they will move to Classroom subscriptions. That means money. Fabulous tool if your district can get it.
Another visual organizer tool that is very basic, but can still be collaborative is Bubbl.us [http://bubbl.us] Partners can be given rights to edit or only read. Registration is required, but is free. Here is a very simple bubbl.us mind map.