This is the classic Greg's Wade's Geared Extruder, revisited in August 2015, with complete OpenSCAD source code including the herringbone (a.k.a. fishbone) gears. It is configured by default for mounting on a Prusa i3 or P3Steel X-carriage with 30mm hole spacing (such as this one: http://www.thingiverse.com/thing:586636 ), and for a J-Head or E3D extruder (original or cheap clone) with 16mm mount diameter.
There are two more remixes available for this extruder, one is the 24mm version for older Prusa i3 carriages, the other is a Bowden version.
The default source code and the included STL files are configured for 1.75mm filament, but this is just an option in the source code and it can be recompiled for 3mm filament.
Note that there is also a complete layer fan system that fits this extruder perfectly, it can be found here: http://www.thingiverse.com/thing:540716 .
The OpenSCAD source code for the extruder itself is based on the latest code by Misan from March 2014, see http://www.thingiverse.com/thing:65939 , with a few small changes.
The OpenSCAD source code for the fishbone gears is a small remix of the excellent, newly rewritten source code by Frans-Willem Hardijzer from May 2015, see http://www.thingiverse.com/thing:818003 .
Printed here in PLA with 0.3mm nozzle, 0.2mm layer height, 50% infill for the body and 60% infill for the gears.
Note that the source code allows for adjusting various configuration options (e.g. gear ratio) and print tolerances, so I strongly recommend checking the OpenSCAD source code and generating your own STL files it for your particular setup.
BTW the included involute gears library by Greg Frost has been updated for the current stable version of OpenSCAD (dated March 2015) and will not compile with older versions.