Hard drives often run out of space, which in addition to lack of file storage, can cause very noticeable operating system performance issues.  When trying to clean up a drive, it is often difficult to find what is the real space-killer.

WinDirStat is a free application that analyzes the drive and creates a visual interactive map of your drive's file usage, allowing you to see exactly what is using up space so that you can make decisions on what to delete.