Diskdeleter - Diskdeleter is data removal software that helps protect your privacy by securely deleting data from your hard drive.
WinDirStat - WinDirStat is a disk usage statistics viewer and cleanup tool, inspired by KDirStat.
Blancco Drive Eraser - Hard Drive Erasure Software
TreeSize - TreeSize tells you where precious disk space has gone to.
CCleaner - CCleaner cleans, speeds up and frees up space on business endpoints.
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.