iGAGING - Snap-Check (Rating: 5)

This type of tool has been available for a long time now. Usually they are cast iron and nearly $200. They are available from chipsfly.com for $20, which includes 3 batteries. I think 3 batteries might be worth the asking price.

This is an accurate tool. Once zeroed, it reads zero each time the plunger returns to the reference surface. Great for setting jointer knives. Magnets in the base attach the snap-check to any cast iron tool surface for hands-free measurements.

I don't find it very useful for setting the height of tablesaw blades (kind of difficult to keep plunger on a single high- ATB tooth), but for the jointer it excels. Also it works as a caliper to measure lumber thickness. Just set it on a cast iron surface like your jointer, and slide a board under the plunger. It reads in fractions, decimals, and mm.
Did I mention it comes with three batteries?

http://www.chipsfly.com/mm5/merchant.mvc?Screen=PROD&Store_Code=W&Product_Code=35-570&Category_Code=