The BPW34 is a tiny, general purpose PiN photodiode. This photodiode has a ton of uses, one of which is to use it is a mini solar cell to power small, low power projects. It is also useful when used as a sensor to detect light. The cell is sensitive to a wide range of light wavelengths (430-1100nm), so it should produce power in a number of different settings. The rated open circuit voltage is 350mV (900nm, 1mW/cm2 light source), and short circuit current is 47μA.
These are fun (of course)! We shined a 940nm infrared LED on one of them and it produced about 0.5VDC open circuit voltage. Stringing four together in series, we were able to turn an LED on with them. They'll also produce a small voltage, ~250mV, in a brightly, fluorescent-lit room.