Simple inline cal tube (1/4" OD 1/8" ID vinyl tube) with 1 cm marks. Easiest way to calibrate extruder. I am sure someone else has this but I haven't seen it.

Just make a mark with a permanent marker at the end of the tube. extrude 100mm or whatever you want and measure the difference from the nearest graduation.

Just thought that I would put this in it's own post so we have a level 1 mod.