The first annual SXSW Music Hackathon Championship took place in Austin, TX during the SXSW music festival. There were almost a hundred hackers, despite the lure of live music all around us. Here are the projects that used the Rdio API:
SongStash — Source
SongStash allows artists to leave their songs at locations for users to listen to when they're at that location. Be sure to checkout the SongStash demo video. Winner, best use of the Rdio API.
A dynamic DJ for parties. Party Play let's party goers create a playlist for the party based on the music people already listen to. Once a party goer logs in to the application their playlist will be polled, normalized, and used to create a play list for the whole group.
Checkout the full list of projects on HackerLeague.