Windows only: Free application Sweet Dreams adjusts the system volume on your PC on a schedule, perfect for letting the music lull you to sleep without leaving it on all night long.
I'm sure it works for other purposes, but this is the most obvious use I can think of—and a use that I personally love. Anyone in the habit of going to sleep to the sweet sounds of their iTunes library has probably also woken up at some point during the night to a not-so-sweet sound from your computer. With Sweet Dreams, just set the timer based on the total duration of the fade, how long you'd like it to wait before it starts reducing your volume, and the baseline volume that you're aiming for.
Sweet Dreams is a free download, Windows only.
Sweet Dreams (translated) [via gHacks]