MindMup is a free, opensource, online mindmapping canvas, Its aim is to build the most productive mind map system online. When you go to the site you are immediately dumped straight into the designing screen, which can be a little daunting. However, the menu bar is simplicity itself and it's very easy to work with. It's free to use, but free maps are deleted after 6 months, and a subscription costs $25 a year.