Regular polygons with genderless hinges, very simple design. Easy to connect and disconnect. By default the side length is 8cm.

I made these after I tried mrule's faces ( http://www.thingiverse.com/thing:5961 ), which were too small for my RepRap to print correctly, and they were impossible to assemble when scaled up. The locking mechanism is copied from the "Polydron Frameworks" product. I've only tried printing triangles so far, faces with more sides are probably not very rigid.

Update: reduced frame and hook width, and added a protrusion on the female hooks so there isn't a hole. The new parts are compatible with the old ones.

The printed parts will need some post-processing where surfaces come into contact to lock smoothly. In particular, use a knife to cut away bits of plastic where the extruder stopped extruding. If your Z axis is set too low, the object will be wider at the bottom - if this happens, cut away the excess plastic at least where it constrains motion.

The source is in OpenSCAD. Parameters have human-readable names; you can adjust all parameters in the declaration of the face() module. The defaults work well for the green PLA plastic I got from FormFutura, which is rather hard. You can get a stronger or weaker connection by adjusting the hook_width and hook_cone_height. The hook_length can also be adjusted, but be careful, because a shorter hook will reduce the range of motion.