Playlistnow.fm is a simple way to find music that's right for what you're doing
Its claim to fame is a search box that starts with the words "I am." (It's kind of like Facebook's old status messages that started with "Jack is ...") You enter whatever it is you're doing (in my case, "I am writing a blog post," although I did try some more NSFW searches just to see what would come up), and the service spits out a bunch of playlists that are supposed to be compatible with that activity, and you can pick and choose.
Playlistnow.fm looks nice, but I must say that it lacks some of the mystique and sex appeal that 8tracks has, at least to me. The interface feels quite cluttered and littered with buttons (each track has no less than seven different buttons for various operations). Still, you get to see all of the songs in the playlist that you selected, and you can edit it on the spot (remove any song you don't like) and skip around from one track to another. These are all things you cannot do with 8tracks, so there's definitely more functionality here.
In the end, it boils down to one question – do you like the music, or not? You'll have to decide for yourself.