Free RPM Program

If you need to check your prop RPM, or anything for that matter, this is a free program that gives you a digital readout of the RPM.  Best of all it is free and it works.  Use it on a PDA or a laptop computer. (Two versions)

Works on sound, so you don't need to get close to the prop.

Get it her for free: 
http://www.tunelab-world.com/rpmsound.html

I use it on a Compaq PDA and the readout is within a few RPM's of my digital tack.