I know of three:
1. Free Mind is a very basic package, but it is free.
2. MindManage from MindJet is the gold standard. I have used it in business for years, and recommend it highly.
3. Personal Brain is new. I have had my copy for only a few weeks, but it is a wonderful piece of software - what MindManage wants to be. But it is like driving a fast sports car, and if you are not familiar with the mindmapping concept, you could get lost in its complexsity. It costs about a $150, as does MindManage. Look at the Personal Brain tutorial which is available on line.
Before you do anything, get a copy of "Idea Mapping: How to Access Your Hidden Brain Power, Learn Faster, Remember More, and Achieve Success in Business" by Jamie Nast. Amazon carries it for about $16.50. It gives a good overview of mind mapping and how you can do it without the software. When you are ready for software, I'd recommend starting with MindManage, and graduating to PersonalBrain, when you get really familiar with the process.
To find the web sites for all three, just put the argument Free Mind, or MindManage, or PersonalBrain in the browser search.
Good Luck. Mind mapping is a really exciting and useful technique.