Soundtrckr is a social music application for smartphones and the web that, amongst other things, lets you geo-tag songs with Google Maps. The application allows you to share songs with your friends via the contacts on your smart phone and also allows you to share song information on Twitter and Facebook.
When you listen to a song you can geo-tag it to where you heard it or to a place that you might associate with that particular track. Then when your friends pass near that location they will receive an alert
Another smart feature of Soundtrckr is the ability to seed radio stations based on tracks or artists that you like. What's more you can even broadcast your created radio station to your friends directly from your phone.