Counting Sheep is a sleep timer for Mac OS X Tiger’s/Leopard’s Dashboard that lets you listen to music or watch a movie while you're already in bed (or leaving the house, but prefer to do so with the approriate music playing). You just enter how many hours, minutes and seconds shall pass before the Mac shuts down and press start. If you change your mind, you can of course stop the countdown later, don't worry.

This widget is simple and simplistic. It's hopefully also self-explanatory: three input fields, for hours, minutes and seconds, one start button, one stop/halt button and one button to reset the whole thing. On its back side you can choose whether you want your computer to shut down or to go to sleep.
Make sure to save all your documents before starting Counting Sheep, because applications asking about what to do with unsaved files will prevent your Mac from shutting down.