xfce4-fsguard monitors a chosen mountpoint for free disk space. It displays 4 different icons (red, yellow, green, unknown) and a message box, depending on the free space. The amount of free disk space is visible in a tooltip. If you left-click on its icon, it opens the mountpoint directory in the file manager.
WWW: http://goodies.xfce.org/projects/panel-plugins/xfce4-fsguard-plugin