Free your mind
I’ve been using FreeMind for about two years. Here’s a good post from a while back, which lays out the main points of why it’s so great.
Effective brainstorming is the art of recording disjointed data (human thoughts) in such a way that they may then be grouped, sorted and prioritised. FreeMind’s tree-like structure encourages the user to start with the big stuff and whittle down to the details – but by the time you’re adding details, they’re falling into groups: making them much easier to manage.