One of my go-to apps for guitar students when we are talking about theory is "Guitarist's Reference." I mostly use this with my more advanced students to relate chords and scales together. It is available online at www.guitaristsreference.com and in the Windows app store, the Apple app store, the Amazon for Android app store, the Chrome web store and on Google Play.
"Guitarist's Reference" is one of the most information-intensive apps I have found for guitar scales and chords. There are several reference libraries (chords, triads, scales and arpeggios) and a chord finder, chord quiz and alternate tunings. All this information can be overwhelming for a beginner, so it is more helpful for students who already have a basic knowledge of the theory behind scales and chords.
It has been very helpful for me as well since I came from knowing the piano where the notes are laid out clearly. When I was first figuring out how to communicate the theory behind scales and chords to a guitar student I would inevitably go to the piano and say "look - it's on the piano like this." Well, that is not very helpful if they do not know about the piano. So this app got me thinking in "guitar" instead of "piano."
There are some really nice features in this app. One of them is that you get to see the chord within the scale that you have chosen. This is great because then students can see where the notes of the chord are coming from in the scale - instead of 2 separate pieces of information, the chords and scales become one. Another nice feature is the chord finder. You can click in whatever notes you are playing to make up a chord and the app will extrapolate which chord/chords that might be. Great for when creating a song or trying to check your work on whether you identified a chord correctly.
There are some downfalls to this app. The biggest flaw is that you are offered many different alternate tunings (woo hoo!) but the chord library will only use the standard tuning (AAARGH!). Having the chord shapes is one of the most useful pieces of information you could possibly have if you are working with alternate tunings. The chord finder, arpeggios library and scale library do work with the alternate tuning so you can still find the notes of the chord, you just have to jump through a few hoops to get it. Also, the only "game" aspect of this app is the quiz. This feature is limited because it will only quiz you on chords, and you cannot select the specific chords you want included in the quiz.
I have had lots of fun working with this app to increase my knowledge of theory and frame my mind in more of a "guitar" mode so I can better help my guitar students. I have also learned about many different types of scales that I did not know existed before!