Buy

http://www.seeedstudio.com/depot/Grove-Digital-Light-Sensor-p-1281.html

Wiki

http://www.seeedstudio.com/wiki/Grove_-_Digital_Light_Sensor

C++ library

https://github.com/Seeed-Studio/Grove_Digital_Light_Sensor

Hardware

Left part cutted and socket removed

Arduino code

#include <Wire.h>
#include <Digital_Light_TSL2561.h>

void setup(void) {
  Wire.begin();
  TSL2561.init();
}

void loop(void) {
  display.println(TSL2561.readVisibleLux());
}