AllOff is a capable free (for personal use) energy management tool which can power down your PC when it's idle.
Windows can do something similar, of course, but the plus point here is that AllOff is far more configurable. It monitors keyboard and mouse activity, CPU usage, download activity and rate. You get to define the thresholds you consider as "idle" (the percentage CPU usage, the download speeds, the maximum time for keyboard inactivity). There's even an option to monitor and control a remote PC.
Once the idle action has been trigged, AllOff can pause for a configurable amount of time, giving you a chance to cancel it. Then, finally, it'll perform your choice of action: Hibernate, Log off, Power off, Reboot, Shutdown, Standby. Although even here you have a further option, as the program can either wait if you have unsaved work, or force any open applications to close.
All of this is presented in a basic but reasonably capable interface. A small dialog box displays your PC's current activity status, highlighting CPU usage and download rates; it minimises neatly to your system tray, and uses little more than 4MB RAM while running in the background.
If there's a small hassle here it's that you must register the program before you can use it. This isn't difficult - we clicked a link during installation, a web page opened, we completed a short form and an email arrived seconds later - but if you're unhappy about giving your email address away then it could be a problem.
Otherwise, though, AllOff is an excellent power management tool which provides a great deal of control over how and when your PC will shut down.