This one was getting a lot of attention around the time of the 3DS announcement for it's faked-3D, which uses the DSi's cameras as motion trackers.
At the time, it was only available in Japan. Now, it's available here. I made the mistake of buying it.

I say mistake because it's broken. The camera mechanic it relies on is so finicky that you may not even be able to play the game. I had to move all around my apartment trying to find a place with a background that contrasts with my face but isn't too reflective and allows me to hold the DS comfortably while still allowing the camera to see my face and on and on. Even then, the 3D mechanic was fidgety and barely cooperated well enough to actually play the game. It jumps around, and leaps from being over-responsive to unreponsive and everywhere in between.

Every time you start the game, the game shows a frame in which you're supposed to align your face. But the alignment the game desires feels awkward, like it's forcing you to watch the game out of the corner of your eye. Then there's a second step, with a sort of light/contrast map (I guess) that you're supposed to use to make sure the system can tell your face from its surroundings. It's almost impossible to get an "ideal" map on this step. This is the one that sent me on a chase all around my house to find a spot that would allow the game to sort of work.

So...Really, really neat concept. But it's barely playable, and even that barely is pushing it.

Youtube clip showing off the tech:
http://www.youtube.com/watch?v=lEMkgVnzvdE
To say it doesn't work anywhere near as well as that video shows would be the understatement of the year.