KDirStat is a fantastic KDE-based utility for managing files on your machine. It very quickly locates your hard drive hogs so you can easily archive or delete stuff you no longer need. Great for freeing up HD space.
There is a Windows port of this, too, called WinDirStat. I like it a lot better than SequoiaView, the first treemap-based visualization program I used.
There is a lot more treemap geekery here if you are interested in the technique.