Web 2.0 is a somewhat nebulous term that among other things covers collaborative websites. Well I just found one that I really like.
Last.fm is a website that helps you listen to music. It records which songs you listen to, and based on that it can recommend other songs you might like, or other people that you might like to discuss music with. Cool! (More info on wikipedia and here )
Last.fm also offers web-radio in which you can tag the songs with "love" or "ban" to further sharpen your musical profile.
Many music players on linux integrate last.fm directly, but on windows and mac OS X you can download a music player that offers all the functions of the service. My favorite music player the amazing Amarok has great integration, as can be seen here