DropMind - picture your thoughts
CMap Tools - users construct, navigate, share and criticize knowledge modles represented as concept maps.
FreeMind - free mind mapping software
MindMeister - online mapping and brainstorming
Bubbl.us - a simple and free web application that lets you brainstorm online.
Connect-A-Sketch - Upload and connect your paper and pencil sketches to create a click-able concept map.
http://www.freetech4teachers.com/2010/03/nine-tools-for-collaboratively-creating.html