Since we recommend shit all the time on here, let's put together a list of some of the essentials.
I'll start with the Korg Pitchblack, because it's cheap, handy as hell and works great. And it has that awesome asian dude's video on YT backing it up. I run it in front of my Axe-Fx II, because it's quicker, easier to see, and acts as cable relief in case I yank the cable out when forging metal. It's also a handy bypass for changing guitars too.
Amazon.com: Korg Pitchblack True Bypass Chromatic Tuner Pedal with 4 Display Modes: Musical Instruments