I made a tiny breakout board for a Melexis US5881 hall effect sensor. A HES detects whether a magnet is near, and is useful for non-contact/waterproof type switches, position sensors and rotary/shaft encoders.
Here is the circuit utilized on the BoB:
The BoB has been sent to OSHPark.com for fabrication. 3 copies cost me $1.40 (shipped). I find that unbelievable.
An US5881 HES is available from adafruit here.
Melexis datasheet is located here.