Have a process running and need to leave before it gets done. Want your computer to shut down while you're gone. Just want a CLEAR reminder that you are done tapping keys and better get on to something else...open a terminal and enter.
sudo shutdown -P TI:ME
your system will prompt for your password and then broadcast a shutdown message telling you how long till shutdown.
try man shutdown for a complete description of the shutdown command and all the cool possibilities.