For Christmas I bought my niece an Eyetoy and a DDR pad. Dance Dance Revolution needs no introduction, but there's a chance that one may not have heard about the Eyetoy. It's a camera that you plug into the USB port on a PS2, and then play games designed for it. Your motions are read by the game, and you interact with it.
If you recall the holographic tennis coach in Total Recall who was teaching Sharon Stone near the beginning of the movie, you're right on track. This is on the TV, but you're kinda in there interacting with the environment with motion. Blue balls fall down the screen that you have to touch (while avoiding red balls), yellow balls pop up that you have to punch to the side, and so on. Streams of blue triangles show up that you have to move through in order. It's pretty neat. It reminds me a bit of the Minority Report interface.
I just did a complete intermediate workout, and I'm pretty worn out. I highly recommend the toy as long as you have enough room in front of the machine to play it correctly. I'm pretty tall, so what worked fine for my 3'11" niece wasn't nearly enough room for me to participate as I was supposed to.
The vision is based on movement, so it's like a T-Rex. If you don't move, it doesn't see you. You show up on the screen, but it can't tell you from the background.
The only minor issue is lighting. I had to install some halogens to light the room up enough for the camera to make out the motions correctly. Also, during one workout I adjusted the camera, it mistook it as my desire to quit the routine, and there was no way to go back to it. I didn't get a full stretch afterward.
Overall, it's a pretty kick ass gadget and I'd recommend it.
The DDR game is enhanced for use with the Eyetoy as well, so in addition to the 4 directional foot options, there is a right and left hand option too. Not only do you have to manage impossible combinations with your feet, you have to remember to put your hands up at the right times. That's pretty neat too, but DDR is tough enough as is, and I like the Eyetoy best with the Kinetic game (the Nike Motionworks game that comes with it).
DDR is a bit more fun than I thought it would be, but it's not as impressive a technology as the Eyetoy.