WinDirStat - WinDirStat is a disk usage statistics viewer and cleanup tool, inspired by KDirStat.
Sirius - An open-source clone of Siri from UMICH
TreeSize - TreeSize tells you where precious disk space has gone to.
Applied Software - Prepare to work with an industry champion! Applied Software specializes in bridging the technology divide from product to productivity no matter your industry.
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.
OneNeck IT Solutions - OneNeck provides a comprehensive suite of enterprise-class IT solutions that are customized to fit your specific needs.