The concept couldn't be simpler. Three columns of dots hurtle towards the "Home" key, and you have to tap them in time with the music. As you nail more notes in succession, your multiplier (and, in turn, your score) continues to climb. You'll occasionally have to use the phone's accelerometer to shake it instead of tap.
There's also a surprisingly enjoyable two-player mode that allows you and a friend to grip either side of the phone (make sure they wash their hands first) and tap the dots now careening toward both ends of the phone.
Besides being free, the game also has some additional songs you can download -- not a bad value-add for a game that was free in the first place. iPhone owners: Give it a shot and let us know what you think. Non-iPhone owners: Please be more dedicated about trying to get an iPhone.