• StartupMonitor
• Phyle's Wallpaper Switcher
• Process Explorer
• Filemon
• Regmon
• TCPView
• Handle
• Cygwin
• PuTTYcyg
• ObjectDock (from Edgar Nidome)
• Startup control panel by Mike Lin
• Google Earth
• SecureFX (not free)
• SecureCRT (not free)
• Google Desktop Search
• Scanner (like filelight)
• WinDirStat (like KDirStat/Disk Inventory X)
• AviScreen (freeware screencast/screen capture tool, used for Time Lapse Digital Painting)
• DirMon2 for executing actions on new files in dirs

See also: http://www.catshoes.com/Software/dsktp2.htm, Visualize your hard drive usage