This is a simple to use light sensor that outputs an analog value from 0 to 5V. With exposure to daylight, this sensor will output 5V.
When the QTI sensor is over a dark surface, the reflectivity is very low; when the QTI is over a light surface, the reflectivity is very high and will cause a different reading from the sensor.
We've had an eye on this Vishay sensor for a while - smart, tidy, and small. Thanks to SparkFun for breaking it out so you can use this to measure ambient light, and physical proximity in a single sensor!