Disk space monitor will periodically check the free space on a disk and can optionally warn you when free space has dropped below the specified percentage.
It can warn in one of two ways. If an email address and server are entered it will send an email every time the check fails.
Alternatively if no email address and server are entered an on screen warning will display.

Disk space monitor can be set to just periodically display the amount of free space by not entering a warning level.

The license of this software is Freeware, you can free download and free use this disk management software.