A review of TREEPIE:

[..]But now there's a new contender called TreePie [4]. Basically, TreePie represents disk usage like a pizza with the size of each slice representing the space taken by each top level folder. Clicking a folder "slice" will then display a similar "pizza" breakdown of the sub-folders and files in that folder. It sounds complicated when I describe it but it's simple and intuitive in use.

It's also highly effective. Add to that the fact that it's also portable, has an alternative Windows explorer view and is free open source software and you have an easy "top product in class" recommendation.
My only complaint is its rather slow scan time. Still, this is not the kind of program you will be using everyday so a short delay is quite tolerable

http://46best.com/dr/best-free-disk-space-analyzer.htm