I just got a Pro membership a couple of days ago, thanks to Random_Canadian's generosity. (Thanks for the hookup!) I figured it was time to quit lurking and actually contribute, so here goes my first Instructable.
As a CNC technician, there have been a few occasions where I've wondered if a spindle or motor shaft was spinning as fast as the motor controller claimed. An old trick I learned a long time ago involved a digital guitar tuner. The trick was to hold a piece of stiff metal shim stock, around .020" or so thick, against a protrusion on the shaft. As the motor strikes the shim it should cause it to vibrate at a frequency that the tuner would turn into a note reading. If you were to look up the frequency for that particular note, some quick math would give you a reasonably accurate RPM.
The problem with this method is that most industrial machinery is not located in a concert hall. The ambient noises in a typical factory reduce even the most careful measurement right back into guesswork. So, if we generate an electrical signal instead of relying on sound, we can isolate and refine our measurement into something useful, and gain a tolerance of around 5 percent.
A small permanent magnet
Enameled magnet wire (pretty fine; mine feels around 0.010")
A digital guitar tuner with an input jack (you can use a good one - we won't be modifying it in any way)
A 1/4" male to 3.5mm female plug adapter
The patience to splice very thin, very fiddly wire
Step 1: The sensor
The sensor is as cheap and easy as it gets. I picked up a package of 1/2" ceramic magnets from Radio Shack for around three bucks. (I buy these five-packs every time I need one magnet, and then the rest vanish. Apparently, they're biodegradable, or I have mice that are far more sophisticated than I would expect.) By wrapping some fine enamel-coated wire around the magnet (see photo) we have a magnetic pickup, like in an electric guitar. It's tough to see in the picture, but there's probably fifteen or twenty wraps of wire on there. I didn't worry about north or south poles, and I don't think that would really have an impact on function.