You get your own online music profile that you can fill up with the music you like. This information is used to create a personal radio station and to find users who are similar to you. Last.fm can even play you new artists and songs you might like. It’s addictive, it’s growing, it’s free, it’s music.
Ooglechaam! (awesome) Stiil caht! (stick it)
I use this with Amarok, a friggin fantabulous Linux music player. It’s sha-weeet. It’s totally cross platform, though, the Last.fm.