This capacitive touch sensor is easily connected to any Arduino compatible using three pins:  GND, 5VDC and Signal.  The Signal pin can be connected to any digital input.  The Signal pin is normally HIGH and goes LOW when someone touches the circle.

Documents:

Datasheet

Schematic

Demo Code:

//This code is an example of the touch sensor connected to pin 2
void setup(){
Serial.begin(9600);
pinMode(2,INPUT);
}
void loop(){
Serial.println(digitalRead(2));
}