Eavesdrop is a developer app for Rdio that turns your friends on the service into virtual DJs. The app lets you select a friend and listen to what they're hearing in real time. As they skip songs or tweak their playlist, you'll hear their changes immediately. It's like listening to the radio, only with your friend in control of the music.
You'll need an Rdio account for Eavesdrop to work, and a few friends you don't mind listening to as well. Once you're logged in, connect your Rdio account to Eavesdrop and select one of your active friends from the list. The player will handle the rest. Eavesdrop doesn't notify your friend that you're listening, but it will add their tracks to your music history, so you might want to give them a heads up why their history looks exactly like yours.
If you haven't tried Rdio, the service recently updated its mobile apps, has a great music library, and you can try it for free.