Another one from Sparkfun, which is quickly becoming a serious UAV parts supplier: the power monitoring sensor board
that Dean Goedde uses for AttoPilot. If you want to add motor current and battery voltage measurement to your autopilot, this is a good way to do it. Analog output, so you'd want to add it to a port with an ADC (ArduPilot has six free ones so it would work well with that).
Oddly expensive at $26, given what's on the board, but perhaps that reflects the low production run.
Woohoo! Now $12.95! (and half sold out immediately). Go Dean!