The FSGuard panel plugin checks free space on a chosen mount point frequently
and displays a message when a limit is reached. There are two limits: a warning
limit where only the icon changes, and an urgent limit that advise the user
with a message. The icon button can be clicked to open the chosen mount point.
The amount of free space is visible in a tooltip.

Open the properties dialog of the plugin (right click on the plugin) to
customize its display.