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.
Bugs should be reported to the Xfce bug tracking system.
The normal (and best) way to get this plugin is to use the package manager or port system of your operating system.
If it isn't available there, or if you want a different version, you can download it in source form from http://archive.xfce.org/src/panel-plugins/xfce4-fsguard-plugin/.