What is Last.fm?
Wikipedia says:
Last.fm is a UK based internet radio and music community website, first founded in 2002. It is one of the world's largest social music platforms with over 325,000 registered users in the US [1] and over 200,000 in the UK and Europe.
Using a unique music recommendation system known as "Audioscrobbler", Last.fm builds a detailed profile of each user's musical taste by recording details of all the songs the user listens to, either on the streamed radio stations or on the user's own computer or iPod. This information is transferred to Last.fm's database ("Scrobbled") via a plugin installed into the users' music player. The profile data is displayed on a personal web page and updated weekly. The site offers numerous social networking features and can recommend and play artists similar to the user's favourites.
Users can create custom radio stations from any of the audio tracks in Last.fm's music library, but are not able to listen to individual tracks on demand, or to download tracks unless the rightsholder has previously authorised it. Registration is required to acquire a profile but is not necessary to view any part of the site or to listen to radio stations.