MK2 is finally here! http://www.thingiverse.com/thing:389006
This is the world's first (mostly) 3D-Printed mechanical endstop switch (that I know of).
Recently I broke one of the end stop switches on my printer, so I decided instead of buying a new one, why not print one instead? This is my first attempt at making a 3d-printable mechanical end stop switch. I put it on my printer, and so far it has worked for dozens of prints. Head on over to instructions for more information on making it.
I can also add parametric files for those who would like to modify this design.

There are two types to chose from: V1 or V2. V1 is the one that I printed and is in the photos, but V2 design has been modified for easier printing and better performance. I haven't tested V2, but would recommend printing it.
The parts that you will need are pretty simple: just some wires, a spring from a pen, some tin foil, and a 10k resistor. (The resistor is not needed if you use internal pullup resistors on your printer's controller electronics.) You can look at the pictures to see how I wired it, it's pretty simple, I added tin foil to the switch's paddle and side to make the contacts, and then I added the wires and resistor to that.
Basically, I tried to make the circut described here: http://reprap.org/wiki/Mechanical_Endstop#Switch_wiring
A simple diagram of my wiring:
GND------------| |-----[signal pin]-----(10k resistor)----+5V