I just thought i would post this because it is a very useful switch.
They sell them at Radioshack for $5, i could not find it on their website but i did buy one so i know that they sell them.
Have you ever needed to use a limit switch but did not need/want the switch to contact with the other part well now you can. The reed switch turns on when it is in close proximity (3/4in or less but it can be increased with more powerful magnets) to a magnetic field. Its works very well and everything is 1/4in in diameter so you can easily mount these in holes.
I plan to use these on robots to determine when two parts are to close together before they collide with each other. I also have some other ideas for its use, if you were to drill a small hole in one of the large wheels and put a small magnet in it you could mount a reed switch on the base so that it lines up with the edge of the wheel. Then when the wheel spins your program can add 1 to a variable every time the wheel makes one rotation in 1 minute which will give you the rpm.
That is a cool way to measure rpm but a shaft encoder could do that same although i think that in some cases this is the only thing small enough to fit in an area.
Here is a picture