Canesta. I looked into their stuff before the Kinect came out. I am no expert in the field, but I thought their time-of-flight sensor is really clever. Basically, the speed of light is too fast to get an accurate ToF measurement within living-room distances using timers. So they use a technique where the intensity of the reflected IR is directly proportional to the ToF. Since you can measure intensity with much more accuracy and precision (and ease) than the time taken for light to travel a few meters, you get a pretty accurate 3D point cloud of the space being sensed. I can't find their white paper that explains this, but it's a good read.