This extruder maximizes travel in X and Z axis, by positioning the motor in lowest possible position on the carriage. The motor is perpendicular to X axis, mounted by two screws tightening it from the top (none of four standard M3 screws are used).
The body is mounted on the carriage only with one screw, allowing very quick detachment/attachment (it's sort of clip-on really). It's also very rigid at the same time - no wobble even at high speeds.
NEW : The carriage can also be used with zMorph DREMEL mount - so you can turn your printer into a milling machine!
(more information on zMorph 3d printer can be found on http://reprap.org/wiki/Zmorph)
Print both extruder_body.stl and carriage.stl. Attach carriage to your X axis using 4 LM8UU bearings (push-fit). Assemble the body : attach nozzle (J-head mount compatible), motor, hobbed pulley on M8 bolt, bearings, guidler (use Greg Frost design), then small and big herringbone gears.
Attach long screws with springs. NOTE : the design has improved, now you can easily attach/detach springs from the top. Another thing : you SHOULD get a hobbed PULLEY, and mount it on M8 screw (between two M8 nuts). It will work with typical hobbed bolt, but it's not recommended.
Clip the extruder to the carriage, and insert one horizontal screw in the bottom part.
Fishbone gears and idler are taken from another design, link+attribution coming soon!