Here is my attempt at designing an extruder for my Prusa i3.

I took some inspiration from different designs to try to make an extruder which is easy to print, compact, easy to clean and has tight guides for the filament (to be able to use flexible materials).

I made a couple of test prints with it and it seems to work fine so far. I haven't tested any challenging materials yet, just PLA.

Here are the components:

• E3D all metal hot end for 1.75mm filament. It mounts like a J-Head so a J-Head should fit as well.
• NEMA17 stepper motor. I am using a 40mm motor but any NEMA17 should work. The back of the motor is mounted on the X carriage and all the extruder parts are mounted to the front of the motor.
• Mk8 Drive Gear. The Idler and filament guide are very tight around the Mk8 gear so other gears will probably not fit without modifications.
• EDIT: The design is now for a 5x10x4 ball bearing and not the initial Yoyo ball bearing (It is a 5x11x5 bearing grooved to 10mm in the center) The groove helps guide the filament but it should also work with a flat 5x10x4, 5x10x5 or 5x11x5 bearing.
• 50mm centrifugal fan (5015S 12V 50x15mm)
• A Spring. I am using one from an IKEA PRESSA clothespin.
• Some M3 and M4 screws and nuts.

EDIT - Version 1.1:
After a bit of testing, the grooved bearing worked fine with PLA and Nylon but not with Flexibel PolyEster so I switched to a flat 5x10x4 bearing. I improved a bit the "idler arm" to make it able to squeeze the filament a bit further and also made it slightly stronger. I was able to make a nice flexible print with this new setup. Stay tuned :-)

EDIT - Version 1.2:
Here is a new version. This is now designed specifically for a 5x10x4 (or 5x10x5) ball bearing and it includes an even tighter filament guide. I had some issues printing flexible polyester with the previous version, this one works better. I have also tested with PLA, Nylon and PET without problems so far.

Here is a video: http://www.youtube.com/watch?v=80q71dtGmuk

I have also uploaded the sources made with LibreCAD and OpenSCAD so you can customize it to your needs. Make sure you upload the remixes! :-)

EDIT - Better fan duct:
Here is a slightly more advanced fan duct which help distribute some airflow around the nozzle: http://www.thingiverse.com/thing:151555

EDIT - Two can fit on a 80mm carriage!
Here is a video of my dual extrusion system: http://www.youtube.com/watch?v=Ca-XfL2IUWs