This is a micro-adjustable bare-wire end stop for reprap printers (or any other machine that could use it). You can also just glue a switch to the top of it if you don't like the bare wire idea.
It attaches to the Prusa end stop holder, and is assembled with a spring, M3-30 screw, nut, and washer. A twisted pair set from an old network cable is exposed atop an adjustable surface and makes contact with another bare wire on the X end.
This endstop is ridiculously inexpensive, and it allows very fine adjustment of the Z stopping position by turning knob at the bottom -- one full revolution is 0.5mm.
I have the OpenSCAD sources for this, and there are actually a number of adjustable parameters. I need to clean the code up a bit and then I'll add it.
You can find my basic "one penny" bare wire end stops here: thingiverse.com/thing:23878