Mindmeister is a favourite tool of mine - I love using it!

If honest, I could probably manage without it, but it’s so simple to use and allows you to get carried away with the important parts of brainstorming, such as generating ideas rather than wasting time formatting text, creating complex structures or worrying about running out of room on the page - it does all that for you.

They offer a basic package that is free to use and if you really like it there’s an application built for the ipad and iphone. I give it 7/10.