OverDisk is a disk usage browser. It can be used to quickly find out how a partition's space is distributed among the file system hierarchy.

To load a directory structure you can select a root path from the Root menu or the first combo box (they both list all the available partitions). To scan only a specific folder, select Root|Any Folder and use the standard Windows folder selection dialog.

OverDisk also adds a command to the shell's menu for items that can be scanned (drives and physical folder). So, you can also right-click on a folder anywhere in Windows and select "OverDisk" to launch the program and start scanning that folder.