WinDirStat - WinDirStat is a disk usage statistics viewer and cleanup tool, inspired by KDirStat.
Zerto - Virtual replication & recovery solution
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.
Macrium Reflect - Macrium Software - the creators of Macrium Reflect backup, imaging and cloning software.
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.
Arcserve UDP - Arcserve UDP is data backup & recovery software that deals with unified data protection solutions, making you protect your computer system by creating data stores on the recovery point server.