The song is Somebody That I Used to Know by Gotye (go-tee-yay). I've heard it once, and can't get it out of my head. Unfortunately, I heard it a week ago, and all the parts of the song are jumbled in my head, so I'm not sure I remember it correctly. There's also the issue of New Song Nostalgia - when you hear a song for the first time and it seems absolutely amazing, but you hear it a second time and it's actually terrible.
I'd have watched the YouTube video five billion times by now if not for the fact that our internet is as fast-moving as half-frozen coffee sludge cross-bred with a giant sloth-slug. I've managed to load the first few seconds once or twice, but it's the chorus that thrills me (or did, if I remember right). Solution? Go to the library this weekend (where their internet is a cross-breed of a bullet train and Michael Phelps) and buy the song.
And I already have the song Architects and Engineers by Guster, but I want more of their stuff. They make me smiley.