Tron Script - Tron is a glorified collection of batch files that automate the process of cleaning up Windows systems.
WinDirStat - WinDirStat is a disk usage statistics viewer and cleanup tool, inspired by KDirStat.
Autoruns - See what programs are configured to startup automatically when your system boots and you login.
TreeSize - TreeSize tells you where precious disk space has gone to.
SKTools - SKTools is a multi-featured and easy-to-use application that is bundled with over 60 tools to help you boost the performance of the system.
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.