Arduino code for capacitive proximity sensors. See it in action here https://www.youtube.com/watch?v=Y2E0ehdnEq4 It requires only a few resistors, paperclips, and alluminum foil.
As your hand gets closer to one, the capacitance increases, and thus the capacitive plate takes longer to charge. The arduino rapidly charges and discharges the plates, measuring how much the plates were charged over one clock cycle.