Developing controller for flying robot I had to research many sensors. Don't have experience with arduino, but maybe those sensors will be useful also for you?Current sensors line: 20A, 35A, 50A 75A
Designed for sensing voltage and current from the battery. Voltage input has 1:16 divider for easy shift calculations, is made with 1% resistors. Current measurement based on shunt resistor and amplifier produce voltage proportional to consumed current. After integration of consumed current we can calculate battery charge level.RPM speed opto sensor
This sensor is used to measure rotation speed in optical way using infra red light. Can works as passive source when external light goes through rotating obstacle like propeller, or can works in active way with own IR illumination. In this case white and black label should be glued or painted on rotating part. Sensor produce impulses proportional to dark and light regions.GPS-GLONASS receiver with optional magnetometer
This module receive signal from American GPS and from Russian GLONASS and predict position using both sources of information. Instead average visibility 6-12 satellites it see 12-24 satellites giving faster position fix and more accurate position. Optionally can be equipped with one of 3 magnetometers: HMC8553, MMC3140, MAG3110. On website you can find C source code for support all those magnetometers.