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 advises 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.
Xfce4-fsguard-plugin documentation
See NEWS for details on changes and fixes made in the current release.
Xfce4-fsguard-plugin source code
Xfce4-fsguard-plugin archive or Xfce4-fsguard-plugin tags
From source code repository:
% cd xfce4-fsguard-plugin
% ./autogen.sh
% make
% make install
From release tarball:
% tar xf xfce4-fsguard-plugin-<version>.tar.bz2
% cd xfce4-fsguard-plugin-<version>
% ./configure
% make
% make install
Visit the reporting bugs page to view currently open bug reports and instructions on reporting new bugs or submitting bugfixes.