Shutdown Timer provides a host of ways to control how and when your PC will be shut down.
You can tell the program to close your system at a particular date and time, for instance, or in a number of minutes from now.
If there's a program running and you want it to run to completion, then Shutdown Timer is able to kick in once your CPU use has been below a particular threshold for a defined amount of time.
It's also possible to monitor network activity, only closing when it drops to a particular level, perfect if you want your system to shut down after it's downloaded some bulky file.
And you can even have your system closed down based on CPU temperature, if you've installed the Core Temp utility (you'll need to tweak the Plugins section of the Preferences dialog to make this work).
Why waste energy (and cash) leaving your PC on unnecessarily? Use Shutdown Timer instead and it'll be much easier to shut down your PC at the appropriate time.