The Bit.Trip series of games are always fun examples of mixing music and gameplay, and Bit.Trip Runner was probably my favorite of these. It is a relatively simple game, where a little black box of a guy named Commander Video runs along, jumping up over obstacles and onto platforms in time with the really excellent retro chiptunes as you play. Bit.Trip has been working on a sequel to Runner for a while now, and Runner2 (full title: "Bit.Trip Presents Runner2: Future Legend of Rhythm Alien") is now available on Steam for the Mac (as well as PC and Linux).
To say Runner2 is insane is an understatement -- this game is positively certifiable. The first game had a very retro feel, but this one is more like a cartoon, full of color and wacky visual gags and crazy figures and creatures. It's narrated by Charles Martinet, for pete's sake, who you may or may not know is the voice of Mario (yes, that Mario, from the Nintendo games).
The core gameplay is still all about Commander Video jumping and sliding in time to the music, but the graphics are so wacky and so wonderful that it's more than enough. As the game goes on, things get even more frantic, allowing you to kick through obstacles, bounce off of trampolines or spin around the joystick (the game is definitely recommended to be played with a controller, which is easy enough on the Mac). You can even make it harder by skipping checkpoints as you choose, winning extra rewards if you go on, but going back even farther if you make a mistake.
Runner2 is an incredible improvement on the original, and takes what was a fun idea and turns it into a gorgeously crafted work of art. The game, I believe, is going to come to iOS eventually, so if you're patient, you can probably wait to try and play it then. But (especially because this version is meant to be played with buttons and a joystick) why wait? You can buy it right now from Bit.Trip directly for US$9.99, or grab it on Steam for $14.99.