WinDirStat - WinDirStat is a disk usage statistics viewer and cleanup tool, inspired by KDirStat.
dupeGuru - dupeGuru is a tool for finding duplicate files on your computer.
TreeSize - TreeSize tells you where precious disk space has gone to.
AllDup - AllDup is a freeware tool for searching and removing file duplicates on your Windows computer.
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.
Czkawka - Multi functional app to find duplicates, empty folders, similar images etc.