Tomahawk Player (www.gettomahawk.com) is a cross platform music (un)locker that enables you to play music from any possible source!
Wouldn't it be cool to drag a Spotify playlist or a Itunes album in to Tomahawk and see it resolve to playable tracks on Rdio?! Yes it would!
Future Possibilities: Rdio could act the same way as Spotify does in Tomahawk, eg. you could have bi-directional Star's to Love's, bi-directional playlist syncing and Collection browsing.
Update: Now you can play full length tracks via Tomahawk