This thing is a support for the z-axis smooth rods of the Prusa Mendel. Supporting the bottom side of the z-axis reduces the side-to-side wobble of the machine and produces surprisingly improved prints. In the image of the large print (part of http://www.thingiverse.com/thing:10526), I placed supports under the smooth rods about 3/4 of the way through the print, you can see the dramatic improvement.
The design uses M6 nuts and bolts to make the support adjustable. Using M6 x 18mm bolts, the feet are adjustable from 24mm - 36+mm. They require 3mm of smooth rod exposed on the bottom of the machine.
I'm open to design improvement suggestions, don't hold back! :)
UPDATE: Added SAE version for 1/4 inch nuts/bolts. A 3/4 inch long bolt is slightly longer than 18mm, so it will produce an adjustment range similar to the 18mm bolt described above. The .stl files for the SAE version have "SAE" in the file name, the metric versions have no designation.
Print a set for each side of your machine. You will need two M6 x 18mm bolts and two M6 nuts. You can use 20mm long bolts if they're easier to find, you will only sacrifice 2mm of the low end of the adjustment range. The hardware fits pretty snug when I print these on my prusa, but it is not necessary that they be snug as they will hold together by compression when in use.
NOTE: The "shorty" version is for those of you who have very little room below your z-axis smooth rods. Using this piece, the assembly will only adjust a couple millimeters, you will need an appropriate bolt to get the proper height.