Tuner: https://f-droid.org/packages/de.moekadu.tuner/ It just does what it is supposed to. There are hundreds of these on the play store, with ads or paid. There is no need for it.
My god this is wonderful!! Thank you so much! It looks like it's only inferior to an in-line tuner, but a little better than those clip on ones, and far better than all the apps that just play a note for you to tune by ear.
Edit: To be clear to everyone, the reason this app is so good is that it uses the mic to measure your instrument, giving you a visual representation of how far out you are.