To draw away customers from its rival Spotify, Rdio is now offering 6 months of free playback to new UK customers. The offer is exclusive to the web/desktop version of Rdio so unfortunately you won't be able to enjoy the long period on other devices. But you can still enjoy 7 days of unlimited playback on your phone or tablet.
The 6 months free does not mean unlimited playback. There will be a bar at the top showing how much free playback you have left for the month. It may not be ideal for music addicts, but it's a good way to try but not abuse the service.