The Servo Expansion has a micro-controller (MCU) that can be programmed to generate Pulse-Width Modulated (PWM) signals. 

It has 16 connectors for PWM devices. It was designed with servo motors in mind, they can be plugged in right out of the box. For the most up-to-date information on the PWM expansion, check out its article in the Onion Docs.

This expansion is ideal for building anything that uses servos: robotic arms, walking spiders, etc, or anything that uses PWM in general like LED lights.

That's what we use to control Oliver, the robotic arm from our Kickstarter video: