The Raspberry Pi is a wonderful computer, but one thing it isn't very good at is controlling DC and Servo Motors - these motors need very specific and repetitive timing pulses to set the position. Adafruit 16-Channel PWM / Servo HAT for Raspberry Pi - Mini Kit adds the capability to control 16 Servos with perfect timing. It can also do PWM up to 1.6 KHz with 12 bit precision, all completely free-running.
Adafruit 16-Channel PWM / Servo HAT for Raspberry Pi - Mini Kit will drive up to 16 servos or PWM outputs over I2C with only 2 pins. The on-board PWM controller will drive all 16 channels simultaneously with no additional Raspberry Pi processing overhead. What's more, you can stack up to 62 of them to control up to 992 servos - all with the same 2 pins!
Adafruit Website: http://www.adafruit.com/products/2327