HALL OMNIDIRECTIONAL ENDSTOP - ONLY TWO COMPONENTS.
After looking at the current hall enstop options, I chose to make my own. I used the Honeywell SS451A. This version is easily built on pref-board with just the hall sensor and one resistor. This is a tested and working on my Mendel Prusa v2.
1) Omnidirectional so you don't need to know magnet orientation. 2) Its a switch; we have no need for ratiometric or linear data, we only need position. 3) It can directly source 20ma - more than enough for most LED's. 4) Through-hole package easy to work with (instead of SMD version). 5) Supply voltage 3 to 24V 6) No external capacitors are specified in the Honeywell installation datasheet. 7) Cost = US$ 1.92/piece, I got 10 for US$ 11.87 or US$ 1.18/piece. (DigiKey part number 480-3587-ND)
1) No internal pull-up resistor, thus need an external resistor (You can turn on the pull-up resistor on the input RAMPS pin in Marlin or Sprinter). 2 When turned on (magnet present) output is low. Magnetic field removed, output is high. This can be compensated for in the software, Sprinter and Marlin const bool (LINE 233 in Marlin; "const bool Z_ENDSTOPS_INVERTING = true;") or could add a PNP transistor to invert endstop output if you can't modify software, but it becomes much more complex).
Here are details on Thingiverse: http://www.thingiverse.com/thing:214825