You can use an application such as LightWave or Call Filter to play MP3's. You can even customize rules for specific instances/numbers. For instance, I had it set that when work calls it plays a clip from the movie Office Space. Hysterical move and a very funny. When my friend calls, I have the Baseball Tonight theme play. Basically, the options are endless. I've used both, but don't have an opinion either way. I've heard that people are real happy with
LigthWave by Toysoft.