This is pretty damn cool:
Korg released
a synthesizer cart for the DS awhile ago and I didn't even hear about it until last week. The cart uses the DS's sound hardware to emulate a sequencer, analog synthesizer, and drum machine.
You use virtual knobs and buttons on the touch screen to interact with the virtual synthesizer. It supports saving 16 16-beat patterns per song, and you can arrange up to 100 patterns to make a complete tune. And of course you can screw around with how it sounds while it's playing.
Here's a cool video of someone messing around on it:
BUT WAIT! You can even wirelessly connect multiple carts and use them all together. Here's a dude doing that:
One thing to make clear:
it's not really a game. There's no goal. It's a little musical instrument.
I ordered this on Amazon because I couldn't find it nearby anywhere and have just started playing around with it. Check it out if you like this sort of thing.
EDIT: think I'll just edit in this other video here