Rob Nero is building this interesting finger tracking device using two infrared lasers and 110 infrared sensors. The cool part about this is that all the math is being done on the microcontroller, which means that it can be used without hooking it to a computer. He writes:
When 2 shadows are detected (one by each laser), it calculates the XY coordinate of each shadow. Using simple algebra, I will be able to calculate the equation for each line that is created (between laser and shadow), and then find where the 2 lines intersect (where my finger is).