I was playing this online about a year ago. It's browser based so you need no software. You are anonymously matched with somebody else online. You hear a music clip and you both have to describe it in phrases to each other (classical, ska, heavy metal, indie, etc). The point is to see if you are both hearing the same piece or different ones. Anybody? This is making me nuts. 
I know it's popular.
I know it's popular.