Pololu customer Alvaro Villoslada made this impressive open-source 3D-printable hand prothesis. Each finger uses a 1000:1 Micro Metal Gearmotor HP 6V with Extended Motor Shaft to wind a fishing line—acting as a tendon—onto a spool. A magnetic encoder attached to each motor enables closed-loop control, and the motors are driven by DRV8838 DC motor driver carriers. An RC hobby servo controls the thumb position. Alvaro uses a Teensy 3.1 microcontroller to monitor the encoders and control the actuators, and he built a user interface in Python for controlling the hand from a computer.
For CAD files, detailed instructions and more pictures and videos, see the Hackaday project page.