Simple Interrupt

#define OUTPUT_PIN 9
#define INPUT_PIN 0  //This is actually digital pin 2!

volatile int value = LOW;

void blinkLed(){
  value = !value;
}

void setup(){
  Serial.begin(9600);
  pinMode(OUTPUT_PIN, OUTPUT);
  attachInterrupt(0, blinkLed, LOW);
}

void loop(){
  Serial.println(value,HEX);
}