One game at PAX that drew some large crowds was Crypt of the NecroDancer. They had the game setup on a large monitor with a Dance Dance Revolution pad and people lined up to dance in this unique dungeon crawler.

The beat drops later this year as a Steam Early Access for PC and MAC and features the music by the amazing Danny Baranowsky(Meat Boy, Binding of Isaac, Canabalt).

If you are looking for a bit more of an explanation of how the game works, Northern Lion posted an early alpha build demonstration that shows it off well.