NightTunes

NightTunes is a sleep timer plugin for iTunes that can automatically turn off, hibernate or send your computer to standby at the end of a playlist.
Unlike other shutdown solutions, NightTunes does not depend on user-set timers to determine when to shutdown your PC.

That way it is guaranteed that your computer won't bail out on you right in the middle of your goodnight tale, only because you forgot to set the timer right.