In the latest post on Digi's Examples site, Matt Richardson shows how to set up an XBee radio to measure ambient light levels. The photocell sensors used are cheap and versatile. You could track solar quality for a greenhouse or transmit information about natural sunlight to run the lighting in windowless environments. And as Matt writes in his post, photocells are not only good for measuring light. You can also use them to detect human presence or even employ them as a touchless controller:
Photocells are a popular component in electronics projects; they let you sense and take action based on the amount of light hitting the cell. Not only are they great for sensing the ambient light to tell if it's day or night, but they can also be used as an interface input. That is to say, if someone waves their hand over the sensor, the shadow of the user's hand can actuate some response. Best of all, these sensors are cheap---at about a dollar a piece, they're a great component to keep stocked in your bins.
Here's a video that shows the light example connected to the panel meter example. Watch it, then make your own!