About a week ago, Ask the Admin ran a feature called What the Heck is Filling Up My Server?. The article was quite a good one and covered several products. I tried a few of these out.
IMHO, the best of these is WinDirStat.
This utility is free and quite fast. There's also some amusing pacman-like activity when you start it scanning a drive.
One of the really cool things about this app is that it recognises the extensions of a lot of file formats - including less common ones like .NSF for notes. An extension that many expensive programs of this type often miss. If it finds a few files of an unknown type, it lists their extension as well (see DES at the bottom of the colour code map).
The other exciting thing about this tool is that it creates a colour-map showing all files in relation to eachother. You can quickly spot the big files. If you click on a patch of colour, the name and path of the relevant file will be displayed in the status bar.
Imagine my horror at finding a couple of gb of MP3s on our system. They've all be removed now so I can't show you that shot. Here's a map with several obvious copies of a large MS Access file.
This software is fantastic for the detection and removal of unnecessary clutter.
The final view that I've found really helpful in this product is a tree-view with bar graphs. It allows you to quickly see where the worst of the clutter is located.
If you haven't given this tool a go, you should.