Analog displacement sensor, measure under 10 microns for under $10! Measurement range of between 0.5mm and 1.0mm (depending on how careful ya are during the build) and a repeatability of around +/- 5 microns. I made it for an application where I was measuring spindle runout with a dial indicator, but wanted to be able to record data to the computer. It's more than a tiny bit larger than the indicator, but I'm quite happy with the performance!

If you'd like full details on the design, build, and calibration, you can find them here.