mArduino is a simple library for Arduino.
mArduino uses multiple Arduinos and I2C Protocol to expand Arduino's IO capacity.

By using mArduino, master arduino can read and write slave's I/O very straightforwardly.
Master can make slave's LED in a nearly identical manner to writing an arduino sketch,
for example, the following sketch will blink slave's LED:

#include "Wire.h"      // You need to include both Wire.h and mArduino.h
#include "mArduino.h"

const int SLAVE_ADDRESS = 1;
const int ledPin = 13;

void setup()
{
  mArduino();            // join I2C bus as a Master    
  pinMode(SLAVE_ADDRESS, ledPin, OUTPUT);  
}  

void loop() {
  digitalWrite(SLAVE_ADDRESS, ledPin, HIGH);
  delay(100);
  digitalWrite(SLAVE_ADDRESS, ledPin, LOW);
  delay(100);
}

For more information, see:
http://coopermaa2nd.blogspot.com/2011/12/marduino-arduino-i2c.html