The song "F**k You" by Cee-Lo Green. It's incredibly vulgar (by some folks' standards), but insanely catchy, musically. You will never, ever, ever hear it on the radio. And yet, not only does unhip me know about it, it's already been listened to by more than a million people since it debuted two days ago, and now I'm going to contribute to its viral explosion by recommending that all of you go to YouTube and watch the video, which somehow manages to be very nearly equally brilliant, despite being nothing more than the lyrics:
http://www.youtube.com/watch?v=CAV0XrbEwNc