This is an interesting DIY tachometer with some unusual features for the R/C enthusiast.
Generally tachometer is a gauge to measure rotation speed of any rotating things. In RC hobby tachometer is the instrument for measuring and monitoring propeller revolutions per minute (RPM). This tachometer has some additional functionality like: thrust, pitch speed and power estimating.
DIY Tachometer – Link
HOW TO – Make a 99000 RPM digital contact-less tachometer – Link