Free Mindmapping/Brainstorming Tools

Wikit - A great place to start.  Lots of tips and tricks about mind mapping

Bubble.us

Skrbl online whiteboard

Mindmeister

Mind42

Gliffy

Exploratree

Drawanywhere

Mindomo 

Dabbleboard

Wordle - Tag Cloud Generator/Creator

DropMind - Mind Mapping and Collaboration Tool

Webspiration

eyePlorer

Creately - create and collaborate online - drawings and diagrams

Glinkr

Visuwords - online graphical dictionary and thesaurus (good for brainstorming)

Imagination Cubed - Draw/brainstorm by yourself, or collaborate with others.  Very cool!

Thinkature - Real-time collaboration for the web

Project Draw

Dabbleboard