A friend recently introduced me to Spotify which is a piece of software you download and install and then you can search for songs, artist, albums, whatever... and listen to them.
You basically add all your favourite songs to your own playlist and then you can listen to them for free. The only catch is that every 30 mins or so you hear a 30 second advert, but you can pay £9.99 per month for a advert free service.
The best bit however is that you can share your playlists with friends and collegues, so for example at my office in manchester we have a shared playlist that everyone in the office can contribute to and then this playlist is played. It means that everyone in the office can control the music and its not just one persons decision as to what music we have on.
It would be great for parties, because during and even before the event people could add their own music to the playlist so everyone gets to hear there own favourite track.
For more info: http://www.spotify.com