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.
Here's another useful sensor for making "Wait...What?" kind of clothing. This LilyPad light sensor outputs an analog value from 0 to 5V. With exposure to daylight, this sensor will output 5V. Covering the sensor with your hand, the sensor will output 0V. In a normal indoor lighting situation, the sensor will output from 1 to 2V. Make a jacket that automatically turns on at night...
LilyPad is a wearable e-textile technology developed by Leah Buechley and cooperatively designed by Leah and SparkFun. Each LilyPad was creatively designed to have large connecting pads to allow them to be sewn into clothing. Various input, output, power, and sensor boards are available. They're even washable!