Easily sense light levels with this TSL2561 Ambient Light Intensity Sensor Breakout Board from Tronixlabs Australia. Features include:
Note! Board can operate from 5V (connect to Vin) however logic (the I2C bus) operates at 3.3V. If your board can deal with 3.3V I2C - great, otherwise order this board instead which has full level-shifting.
Arduino library - click here.