Hack Quebec, hosted in beautiful Quebec City, was supported by the local government and featured local government APIs. Here are the projects that used the Rdio API:
iPhone applications lets you pick three albums from an artist and receive recommendations. Uses the Tumbz API to improve the recommendations. Winner, best use of the Rdio API.
Find local concerts for artists in your Rdio collection
Inspired by chatroulette. Meet random people, trade-off playing songs with each other. The listener can give you points for the song if they like it.
Turntable.fm-style listening rooms
OS X menu bar application that displays what you're currently listening to on Rdio