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.

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

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.

Baobab Disk Usage Analyzer - Baobab Disk Usage Analyzer is one of the light-weight disk analyzers that offers you a chance to view and monitor the disk usage & folder structure without any hassle.

JDiskReport - JDiskReport enables you to understand how much space the files and directories consume on your disk...