SpaceSniffer - SpaceSniffer is a freeWare (donations are welcome) and portable tool application that lets you understand how folders and files are structured on your disks.

WinDirStat - WinDirStat is a disk usage statistics viewer and cleanup tool, inspired by KDirStat.

Scanner - This tool uses a sunburst chart to display the usage of your hard disk or other media.

WizTree - WizTree quickly finds the files and folders using the most space on your hard drive. It scans the MFT (Master File Table) instead of crawling the entire disk which makes it very fast.

IntoDisk - IntoDisk is a tool that helps to analyse your disk space and explore it easily.

TreeSize - TreeSize tells you where precious disk space has gone to.