The Park Chain Checker CC-3 is a neat little tool that can save you some cash. The tool is fairly cheap at less than $10. The point of the tool is to measure chain stretch so you replace your chain before it stretches too much and you end up prematurely wearing out your cassette or all your hard earned cash on a new pricey 10 speed chain…
It works pretty simply, you drop it in the chain, one side is for 0.75% stretch and the other is for 1.0% stretch, if the tool fits in between the links, it has stretched. I dig it, it can save you some cash. I picked mine up at my local REI.