Musicovery is a very nifty little music search engine that lets you search for music to suit your mood, and by genre.

And it lets you listen to whole tracks, free, and skip instantly if you don't like something.
Not a massive selection of tracks, and with no way to upload your own playlists, it's a nice but limited idea.
The naviagation is interesting, but lacks full scrollability, it would be better if you could browse down the music 'branches'.
Basic business model in that there are simple links to itunes and amazon music for the track that is playing.
Musicovery isn't 'intelligent', and does serve up a pretty random selection of tracks. I rather like that, but if you fancy something more personalised then Last.fm claims to be leading the 'social music revolution'.
You type in a track you like and it serves you up something similar. You can rate whether you like it or not, and it'll learn your preferences. It crosses search with social networking, in that you can create a profile, and see what your friends are listening too.