Meet the uArm Pro: a 4DOF metal robotic arm based on Arduino that teaches you a thing or two about robots and programming. You can use it for 3D printing, laser engraving, writing, painting, and everything in between. This open source robot arm has a strong, lightweight structure.
You get a drag & drop interface to interact with the robot. An offline learning mode is also available. The robot arm has a 500g payload capacity and repeatability of 0.2mm. More info is available here.