This is a very small light sensor. A photocell changes (also called a photodetector, photo resistor, CdS or photoconductive cell) resistance depending on the amount of light it is exposed to. These little sensors make great ambient light triggers (when light in the room turns on, do something).
An assortment of the most common sensors used in electronics. 9 sensors and 3 additional components to interface with the 'analog world'. We have some tutorials here, to get you started.
Triple-axis analog accelerometer - for measuring motion and tilt
Force sensitive resistor - for s..