A friend introduced me to this today, If you live in the UK definately take a good look at it.
Free - Spotify
This is basically an iTunes with a massive library, you just search it and make playlists based on your music tastes. I'm really impressed with this, its all free and legal.
You don't download it, its streams (seamlessly i might add) and becuase you login to it you can get your playlists up on any computer with spotify on.
Its gonna be great for parties because i most certainly don't have the same taste as all my friends so they can put on the music they want.
I'm quite excited by it so wanted to tell you guys, almost feels like the start of the evolution (not quite dramatic enough to be a revolution lol) that needed to take place in the music business.
edit: the free version obviously has a few adverts, it really is only a few. much much less than the radio