Android: Using your phone to play music as you drift to sleep is great. Finding out you left the music app running for eight hours is not so great. Super Simple Sleep Timer solves this by killing or quieting your apps after a set time.
The app allows you to choose either specific media apps, such as your music player, and set a timer that will force them to close after a set number of minutes. Alternatively, you can select "All" to close everything at once. The timer is fairly spartan, but it gets the job done.