Eavesdrop is a new third-party web app that utilizes Rdio's API to let you listen in on a friend's playlist. By simply signing into the app with your Rdio credentials, users can pick from any friend currently online and instantly tune in to their current selection. From there you'll hear what they hear, though the person on the other end is never alerted that their listening habits are being monitored. That could draw some privacy concerns, though the invasiveness is rather limited since you presumably know your Rdio friends — or at least don't mind them piggybacking on your session. Your songs will still be shared via Facebook if you've enabled that setting however, making it possible for a friend to uncover your eavesdropping ways.