The VEML6030 is a high accuracy ambient light sensor with 16-bit resolution. Even more impressive is that it can detect light similar to the human eye response.
The SparkFun Ambient Light Sensor reads ambient light in lux and boasts a number of nice features including: the ability to set high and low thresholds for an optional interrupt, power saving features that enable single digit micro-amp current draw, and a readable range from zero to 120,000 lux. Sparkfun has written an Arduino library that gives full access to all features and includes example code demonstrating all its abilities.
More information is available on the website of Sparkfun.