Update: A hardware kit is available here

This is a lightweight belted extruder derived from the geometry of Greg Frost's excellent hinged accessible extruder.
Design elements:

1. NEMA 14 motor. This reduces the moving mass to about half that of a standard NEMA 17-based extruder.
2. GT2 (2mm pitch) belt drive. The belt is efficient and quiet. It eliminates the clicking noises that printed gears make (the real reason I started this project). The belt should also have a longer wear life than printed gears. This means that it requires a small GT2 pulley and belt, but the large pulley can be printed.
3. Standard Greg's Accessible Extruder geometry. The overall frame is shorter than the GAE, but I elected to retain the 608 bearings, 8mm bolt and Greg Frost's excellent guidler.
4. Additional frame structures to completely capture the hobbed bolt bearings. This allows the frame to hold the thrust loads generated by the belt, which are the opposite of those in a gear driven extruder.
5. Standard 8mm or 5/6" hobbed bolt. Hobbed bolts are available here: http://clough42.com/go/product/hobbed-extruder-bolt/
6. Uses all metric hardware. The motor is mounted with M3x10 bolts, the guidler pivot is M3x25, the spring tension bolts are M3x50 or M3x60 and the nut traps for mounting the extruder are M4.
7. Compatible with groove mount hot ends. My initial testing is with a 3mm/.4mm Hexagon.
8. Self-supporting for printing on single-extruder printers. The STL file includes additional structures and bridging that can be broken off after printing. Your printer will need to be able to bridge about 17mm to print this successfully. A version of the STL without the support structures is also provided.

See the instructions for details about the vitamins I used.

A video of the extruder in action is here: https://www.youtube.com/watch?v=0NzpV7vTFIg