FreeMind excites me because it illustrates the flow of my thinking. FreeMind is open-source mind-mapping software, and the download is free.
It's going to change how I do everything--plan projects, outline papers, and study for my Game Theory class next semester.
You can open up a mind map to editing by by a team, and there are handy icons to help you prioritize items in your map.
There's even a smiley face icon to show when you're happy and you know it.