Music Hack Day once again returned to Rdio's home territory of San Francisco. Like last year, it was hosted at ToxBox HQ. Checkout this year's projects that used Rdio:
Follow Swap — Website
Get out of your following comfort zone by following someone random. Agree to follow a random user on Rdio and receive a random follower later. Maybe you'll discover your new favorite band.
A reddit-style voting system for listening to music with friends
Soundtracking + Rdio
Second-screen player for Rdio. Includes listeners from soundtracking who have liked, loved, or listened to the track