There are some commercially available tachometer’s but why pay for one, when you could built it and have more fun. If you don’t know what a tachometer is, in shot words its an instrument that measures the revolutions. In our case, in rc hobby it measures the revolutions of a propeller (revolutions per minute). This tachometer has some additional functionality like: thrust, pitch speed and power estimating. Its based on ATmega8L, LPH7366 (Nokia 5110) LCD and can measure between 100-99990RPM.
RC hobby tachometer: [Link]