Here is an interesting machine which is part of the HackaDay Prize competition. It's created by a man named Tyler Anderson and it uses a polar coordinate system of printing. The machine, called Theta uses 4 extuders printing on a rotating circular base. The extruders can all print simultaneously, making this machine quite the work horse. More details on this amazing 3D printer can be found here: http://3dprint.com/13310/theta-3d-printer

Below is a picture of the incredible machine. What do you think of the Theta 3D Printer?