My extruder design for my delta printer build. I'm using a Nema 17 stepper motor with a 5.1 planetary gearbox, which works really well. No skipping steps, no grinding filament!
In addition to the motor it uses a MK7 hobbed gear, two 10x3x4 mm bearings, four M3 screws, a M4 screw, a M4 nut, aM4 bowden coupling and a 10-15mm spring.
I had to print a washer and put it onto the gearbox in order to fit everything nicely, you might not need it. The bottom part is used to attach the extruder to aluminum extrusions (or something else!), and the knob can be used to manually extrude filament.
Here is a link to the complete printer: http://www.thingiverse.com/thing:923830