Meet the Delta X: an open source delta robot that can be used for 3D printing and assembly tasks. It can be combined with various end effectors to take on a wide variety of tasks, such as drawing, picking & placing, and testing touch screens. It can be controlled using a computer, smartphone, and other devices using a wired or wireless connection.
Delta X is based on Arduino. By playing around with it, you will learn more about C++ too. The robot has a max payload of 500g and joint speed of 130 degrees/s. You can find it on Kickstarter.