Jordi figured out how to make the ArduPilot shield $5 cheaper--now it's just $32.50 with the differential pressure sensor.
--Battery voltage sensor
--3.3v power regulator to support 5Hz GPS modules
--Allows programming of ArduPilot without removing GPS
--Easy attachment of other sensors and wireless modems
If you've got an ArduPilot, either 168 or 328, you totally need this. Flying with an airspeed sensor is 100% better!