LEDs are great for making light, but they can sense light too!
I recently stumbled upon this site
which mentions, "In 1977, Forrest M. Mims reminds us in one of his "Engineer's Notebooks" that LEDs can also be used as photodiodes... " The page features a LED matrix being used as a multi-touch input but offers very little information. I still have no idea how to make his project, but I did learn how to use LEDs as light sensors.
This instructable goes over my experimenting with different kinds of LEDs as light sensors and making a dark activated night light using LEDs as sensors.