If you've never tried mind mapping software before, point your web browser at www.mindmeister.com to try it out. There's no software to install or download, and a basic account on the service is free. You can create up to 3 maps, and you can even collaborate with other users online.
If mind mapping isn't something you use often, but would like to have easy access to, then maybe this browser-based version is all you need. It really is very simple to use, and you can get started instantly in demo mode without even signing up. Though you'll need to create an account before you can save your work.