Most all of the online, brainstorming tools now are free. Some do ask you to register, but it isn't required. I like being able to work with others and see the changes and additions as they happen.
Here are links to a blog post of the Top 5 Web 2.0 Brainstorming Tools and take a look at the Brainstorming page in our Student Online Collaboration livebinder. Have fun brainstorming on one of your favorite topics and let me know which program you liked the best or share one of your own.