This app allows you to easily select any song on your phone as a ringtone.
To use it, go to any app that uses a ringtone. Select RingTunes from the list of options, find your song, and press OK.
For the security-minded (or the curious), see the source code at https://github.com/peteyblueeyes/RingTunes
Original music graphic at https://upload.wikimedia.org/wikipedia/commons/3/35/Simple_Music.svg