Calando

Often fall asleep listening to music, but don’t want it playing full volume all night. So, Calando is a small OS X application wrote to gradually fade the volume down to some preset level.
Calando has only two options, ending volume, and duration.

The ending volume slider allows you to select what the final system volume will be after the time specified in the duration field.
The final volume selection can range between mute (all the way to the left), and no change (all the way to the right).