A colleague recently told me about Pandora Internet Radio, an extremely cool service that builds a personalized streaming radio station based on your listening habits. You tell it the artists you like and then based on this it plays music it thinks you might also enjoy. It does this by identifying various characteristics of the music you like such as tempo, rhythm and melody, through a project they describe as the Music Genome Project.
The service is currently supported by adverts and completely legal, but sadly only available in the US at this time. However they say they are working on sorting out the licensing issues in other countries, so I'm really looking forward to seeing that over here in the UK and giving it a try.