Yahoo messenger lets you listen radio and you can check out other Yahoo! members who like your style of music.

With Google talk you can let your friends see what you’re listening to by selecting “Show current music track” in your status menu and check out what Google Talk users are listening to across the world. Check also Google Music Trends.

Last.fm is a website (with an associated program) that tracks what you listen to, learns what you like, and suggests music you might like. These preferences are also extracted from your social network and from people with your similar music taste.

All of these programs have more or less the same features and propose the same concept. The scope is clear, offer users the music they like, and eventually, monetize the service somehow.