Simple, Free Drive Space Monitoring With Tray Disk Free
The 29k Windows only app goes resident in your system after launching and monitors the free space of any drive on your system. You'll barely notice the 1mb of ram it consumes.
Click the tray icon and Tray Disk opens its main window behind your other apps. Its taskbar button gives you a full display of free bytes on your drive and a percentage of total drive space.
Tray Disk supports command line arguments that allow you specify the drive to monitor, color of the display bar, and the number of seconds between updates. You can spawn multiple instances to keep tabs on other drives. It works well in the system tray but overwhelming on your taskbar. I'm guessing that the tray indicator will provide enough detail for most users.
Tray Disk is released under the GPL, and is for Windows only. Vista users will need to drop MSVBVM50.dll in the same folder as Tray Disk's executable.