A cool website is www.skrbl.com.
Technically it's an online whiteboard, but you can use it as a brainstorming tool.
Basically, you can invite users who are online (on a computer in the classroom works as long as they can get out to the internet) , and everyone can edit the "white board" at once.
Cool activity that encourages writing. I saw this in action with a Smartboard, it was really neat.