LikeTones, a music-education company, offers a guitar tuner for Android and iPhone that's both free to use and free ... which I found reference to online, seems to be missing from Google Play.

[Paul Gallagher] has the answer in one case, he’s used not one kit but two of them as for a guitar tuner project. At its heart is a Boldport Cuttlefish ATmega328 development board, and for its ...

And while some guitar tuner apps offer chromatic tuning, others will only give you the basic, six-string, EADGBE tuning notes in their free version. That might be enough for some of you ...