Artists use this
Still working, 40 years later
Released in 1978 (maybe earlier?), but I got mine free with a guitar I bought from my uncle. Still works in 2019, which says something about Korg's build quality then. There's a simple plastic cover for the 9V battery (no messing with screwdrivers), a button to test the battery level without removing it, and a simple light to illuminate the tuning display. You can also tune it via the built-in microphone, if you're too lazy to plug in your guitar. The main downside is that you have to manually select which string you're tuning - this isn't some 2019 VST Plugin that automatically detects whatever note you're playing down to 1/100th of a cent. And if you're way out of tune - like I usually am - the needle won't even move until you're within +/- 50 cents of the note. So it can sometimes look like the unit is broken, but really you just have no sense of pitch & are a talentless hack who shouldn't be playing your uncle's guitar in the first place. It comes with a convenient wrist strap, for some reason I don't quite understand. But seriously, it does the job, and it's nice to see electronics that still work 40 years later... and not a single software update required.