Bubbl.us is a simple, free web application that lets you create mind maps for organizing and brainstorming ideas. Mind maps are great for visually laying out ideas and seeing the connections between them.
Here is an extremely simple map that I did.
As you can see, you can also get much more complex.
Bubbl.us is an extremely simple to use application. Simply hit enter to create a new main topic and tab to create a subtopic. Use your mouse to move topics around and change their relationships between each other. You can also change the color and size of the topic bubbles.