A Steinway grand piano works for me.

I second this the Michael Moessner one and it’s foss ! Also available on the play store : https://play.google.com/store/apps/details?id=de.moekadu.tuner

The same developer proposes a metronome : https://play.google.com/store/apps/details?id=de.moekadu.metronomenext

I have an old pitch pipe. If I can’t tune by ear I get out the pitch pipe and blow into a certain note that matches the particular string. I then do my best to make them sound the same. I bought it once, back in the 80’s, and it still works!

For Android I use Moessner’s Tuner, it’s precise and responsive.
On PC (Linux) I use Chromatic or the Guitarix team plugin if I’m working on a DAW.

Tuner in F-droid is good. I’ve used several different android based tuner apps, and a couple Linux based tuner apps over the years and they’ve generally all been serviceable. Tuner is what I use at the moment.

I use VocalPitchMonitor on android. It’s free and just gives me a dead simple line showing pitch. It’s not specialised for guitar at all, but all we need to know is which note the string makes when we pluck it

I use ccgt on F-Droid.

I use insTuner on iOS. It’s a basic tuner, but seems to work well for all my instruments. It has a tone generator too, in case you prefer that method.

Spectroid or Voval Pitch Monitor on Android playstore:

https://play.google.com/store/apps/details?id=com.tadaoyamaoka.vocalpitchmonitor

I use Cleartune on iOS. Simple and accurate. Been using it for years.