After some research, here are a few ideas:
- http://bubbl.us/ - is a "free" site that allows students to brainstorm with easy to use features. One disadvantage is that it requires a username and password to save. You may want to even consider a "screen shot" to save a bubbl.
- http://www.dabbleboard.com/ - is another "free" site that requires a username and password. There is a paid version of the software too. A great feature of this site is that you can download your brainstorm to your computer as .png file.