Playlist streaming site. Pick some genres, name some favorite bands, and -- presto -- excellent playlists of all sorts and sizes. Extremely good for passing time at work or finding music similar to X.
Made one of these a few years ago for my girlfriend, didn't know it was still around:
http://8tracks.com/hissnlissn/some-songs-to-hear-tomorrow-my...
edit: doesn't appear to work, probably due to the fact that it's relying on youtube and the playlist is 3 years old. spotify wins again.