Despite usually dismissing these things as the domain of idiots who wear Taz ties and say things like ‘on-message’, I’ve found that FreeMind is actually a rather useful tool.
It’s a general purpose brainstorming program that creates spider diagrams and the like. It’s free to download and use and seems to be available for all platforms (Mac, Windows, Linux, etc). Handy if your ideas have a tendency to disappear amidst a sea of non-stick Post-It notes. It also does all sorts of whizzy things like exporting to HTML for inclusion into websites. The only caveat is that you need to have Java installed in order to use it.