Micro servo - SG92R
Tiny little servo can rotate approximately 180 degrees (90 in each direction), and works just like the standard kinds you're used to but smaller. You can use any servo code, hardware or library to control these servos. Good for beginners who want to make stuff move without building a motor controller with feedback & gear box, especially since it will fit in small places. Of course, its not nearly as strong as a standard servo. Works great with the Motor Shield for Arduino or by just wiring up with the Servo library. Comes with a few horns and hardware.
To control with an Arduino, Adafruit suggests connecting the orange control wire to pin 9 or 10 and using the Servo library included with the Arduino IDE (see here for an example sketch). Position "0" (1.5ms pulse) is middle, "90" (~2ms pulse) is all the way to the right, "-90" (~1ms pulse) is all the way to the left.