Well, dfolder doesn't work with Windows 2K or XP and I have to say that I don't really like the user interface with Spacemonger (although the price is right!).
The one I like (but it costs) is bundled with PowerDesk 5.
www.ontrack.com It is called Size Manager, and it lets you see the entire tree or a subdirectory tree, which can be sorted by large to small. I use it to do my purges of old, humongus stuff clogging up the arteries of my hard drive. Once you find a bloated directory you can right click on it to explore it.
Typically I end up burning CD's and moving stuff off. (DVD's soon)