JDiskReport is one of those few great tools that I have come across for Windows. JDiskReport is a simple java based utility that can scan your Hard Disk or even a folder of your choice and analayse and report on the space used and the range of other information like the ranges of file sizes, range of files last modified etc. JDiskReport enables you to understand how much space the files and directories consume on your disk drives, and it helps you find obsolete files and folders. All these nicely shown in Pie-charts and graphs.
JDiskReport is free and can be downloaded and installed or run as a webstart (java)