WhatSize will summarize your disk usage on your (Machintosh) disk in such a way you can quickly zoom in on the biggest offenders and get rid of that stuff you don't really need that's taking up all your disk space. This is very useful if (for example) you've had your new MBP for a few months now, and are finally starting to get low on disk space.

WhatSize is marvelous. In my case, I was able to quickly identify the source of many GB of disk usage on my disk--my trash folder. (Not to self, remember to empty the trash every once in a while).