FosiX and SpaceMonger are pretty good free windows disk space usage analyzer's, but if you are looking for a simple and fast pie-chart of your disk usage, Give open-source utility "Scanner"a try.

The small (just 164 KB) free utility creates a concentric pie chart displaying disk space usage on your computer media, The chart shows all major files and folders from all directory levels at once, the utility also provides a simple explorer extension to perform quick analysis of a folder.