SleepyTimer

SleepyTimer is a simple, uncluttered menu bar application that allows you to set a timer and have your Mac go to Sleep, Shutdown, Restart or Log out when the timer runs out.

Useful if you are doing a long download or listening to music and want your Mac to not stay on for the whole night.

You can choose to have a window sit in the background of your screen with the countdown on it to remind you.