If you're a home-theater-PC pusher—or if you just like to watch TV and movies on your small screen—using the automatic shut down feature can act as a sleep timer and save tons of energy.
Even though watching TV before bed is ill-advised, we all need a lazy night sometimes. Before you hit the sheets with your laptop tuned into Hulu, you might want to set a computer "sleep timer" to keep your eyes closed and your pockets full.
If you're working on a Mac, you've already got the necessary application built right in. Head to System Preferences and set the Energy Saver option to shut down the system at a pre-set time.
If you want some added features, check out Smart Shutdown for Windows or iWannaSleep for Mac. Those applications will shut down your machine and also clear the recycle bin/trash can and quit any applications before it dozes off, if you ask it nicely.