The TSL2591 luminosity sensor is an advanced digital light sensor, ideal for use in a wide range of light situations. Compared to low cost CdS cells, this sensor is more precise, allowing for exact lux calculations and can be configured for different gain/timing ranges to detect light ranges from 188 uLux up to 88,000 Lux on the fly.

Approximates Human eye Response
This board/chip uses I2C 7-bit address 0x29 and 0x28 (Both)

Resources
A detailed tutorial showing how to wire up the sensor, use it with an Arduino and example code that gets readings and calculates lux.
Datasheets, Fritzing object and PCB CAD files available in tutorial