Capacitive Sensor (CapacitiveSensor.h)

http://playground.arduino.cc/Main/CapacitiveSensor -- The capacitiveSensor library turns two or more Arduino pins into a capacitive sensor, which can sense the electrical capacitance of the human body. All the sensor setup requires is a medium to high value resistor and a piece of wire and a small (to large) piece of aluminum foil on the end. At its most sensitive, the sensor will start to sense a hand or body inches away from the sensor. Version 04 adds support for Arduino 1.0, and fixes an obscure possible race condition with Tone, Servo and other libraries that perform I/O in interrupt context. Version 03 has been updated to C++ and supports multiple inputs. It also includes some utility functions to make it convenient to change timeout values.