I am a big fan of Mindmapping software. For those people who do their best thinking standing in front of a whiteboard with marker in one hand, and eraser in the other, these applications make me more productive.
I started with Freemind (freeware), then upgraded to Mindjet (~$600), and not think I have found the next successor.
MindMeister is still in beta but it looks to be very promising. I could write more, but it's probably easier to map:
Note: they are also embeddable via an iframe directly into blog posts, but it was breaking my layout, so I just did the direct link instead.