Windows only: Free application TrayDiskFree is a tiny executable that adds an icon to your system tray displaying the free space of any drive you choose—and that's about it. If TDF doesn't mesh well with your taskbar, the app supports command line options for customizing the color as well as specifying which drive to use at startup. File this one into the lightweight, single-use application bin. If you want more full-featured system monitoring, check out previously mentioned apps like Samurize or CoolMon.