Nothing still quite comes close to Pandora. I still remember the day I tried to login, and all I got was a message saying that it was no longer available to non-U.S. residents. Anyway, there have been others, such as Grooveshark, but none of them are able to recommend artists and songs based on more than just genre. Pandora looked at vocal styles, lyrical content, instrumentation... Every recommendation was surprisingly good. (The key word here is "surprising".)
Anyway, Youtube has just launched "Disco". It doesn't quite list all of musical elements of each song the way Pandora did, but it does let you do things that others didn't:
1. Watch the music videos.
2. Replay any song any number of times, as desired.
For these two reasons alone, so far, it wins.