'LikeTones' Is a Guitar Tuner With No Ads or Upsells
Tune up without getting bogged down.

I've been trying to get back into playing guitar and I've noticed one thing: Both the App Store and Google Play are replete with guitar-tuning apps that are full of either upsells or advertisements. Isn't there any application that helps tune your guitar without bothering you?
It turns out, there is. LikeTones, a music-education company, offers a guitar tuner for Android and iPhone that's both free to use and free of ads and upsells. You can open this application, play a string on your guitar, and start tuning. The application automatically detects which string you're trying to play and tells you whether to tune up or down.
If you'd rather not use this automated tuning, or find that it doesn't quite work, you can tap a string to tune it specifically. This also plays the note from your phone's speakers, giving you an idea of how the string should sound.
Because not every guitar is the same, this application offers support for six, seven, eight, and 12 string guitars in the settings. There's also support for various alternative tunings. You can also configure the reference frequency, if you want—this is useful if you need to tune to a piano or other instrument that isn't quite standard.

It's a well-thought-out application, and the best I could manage to dig up in my capacity as an amateur guitarist and professional app recommender. It even has a built-in metronome.
There are other solid options out there. Jon's Tuner for iPhone also works quite well, and is clean, but doesn't offer the same flexibility in terms of alternative tuning schemes. The Android version of Jon's Tuner, which I found reference to online, seems to be missing from Google Play. There is a solid Android alternative, though: Tunerly, which is completely open source. This app offers a simple user interface and settings, allowing you to choose specific tuning schemes.
For most people, though, LikeTone Guitar Tuner is currently the best option. It works on both Android and iPhone, it's clean, it works well, and it stays out of your way. Try it if you're even a little unhappy with your current guitar tuning application.