KeyOff

KeyOff monitors keyboard and mouse activity and initiates computer shutdown if there has been no keyboard or mouse activity for a specified time.

It can also be used to control a remote computer. KeyOff supports all the normal shut down modes.
KeyOff is part of the JSutils group of software utilities which have a theme of energy conservation.
To find more energy and money saving ideas check out Practical Home Energy Saving.
Uses of KeyOff include shutting down a computer when it has been idle for longer than a specified time.
It can be started via the Windows Task Scheduler, on startup or manually.
KeyOff is a great little utility for saving energy by making sure a computer shuts down when left unattended.