So you're chasing a misfire and not sure if it's the coil or the plug?

It's unbranded – just a generic tool – but for the price, it does the job.
There's no manufacturer part number or UPC, and it's made in China.

How it works: you unplug the coil, plug this tester in, then plug the tester into the coil.
Crank the engine and watch the spark – you can see if it's weak or inconsistent.
The adjustable gap lets you simulate different combustion chamber pressures – wider gap means harder for the spark to jump.

Great for DIY mechanics who want to verify spark before throwing parts at a problem.
Works on most gasoline engines – cars, trucks, small engines, you name it.
Cheap insurance against buying a coil you don't actually need.