CdS (Cadmium Sulfide) photo-resistors are fairly cheap and widely used light detectors. Since CdS cells contains Cadmium, a toxic heavy metal, has now become hard to find. The reason is because of the RoHS directive.
Electronics-lab has reported an excellent alternative for CdS that is more versatile and cost-effective. The idea is that when you are building a micro-controller based circuit such as a PIC or Arduino, then you can use a simple LED to achieve the same performance as CdS or even more.
To learn how you can use LEDs for light sensing, you can visit Scott Daniels post at Provideyourown here.