So, I wanted to make it easier to level my print bed (Mendel Prusa, mounted on 4 x M3 bolts with springs), and the obvious answer was to install thumbwheels to make it easier to adjust each levelling screw quickly and easily. Sure, I could have just downloaded someone else’s design like this http://www.thingiverse.com/thing:13807 or this http://www.thingiverse.com/thing:5246 or this http://www.thingiverse.com/thing:13065 – but where’s the fun in that? Far more satisfying to design and print your own!
So I created a design in Alibre, exported the STL, and printed a set of 4 (in a single nested print). (I have uploaded the Alibre file for anyone who wants to modify the design for other applications; sorry, no OpenSCAD files, but it shouldn’t be too hard to design something similar in OpenSCAD!)

Print a set of 4 (for standard Mendel Prusa print bed); clean up if necessary, press in M3 nut, and install in your levelling screws for easier fine-tuning of the height and level of the print bed.