> The most useful tool I've found is Perfect Ear for Android and iOS. It's not as full-featured as GNU Solfege, but it covers all the important stuff and it's far easier to dip into when you have a spare moment.
Oh man, I missed this app when I switched from Android to iOS. So glad it's finally on iOS!
https://www.perfectear.app/
If you're a late beginner/early intermediate player and trying to move past the chord shapes and scale patterns rut, I strongly recommend William Leavitt's A Modern Method for Guitar. It's hard - really damned hard - but it's logical, the difficulty curve is fairly linear once you get past the shock of traditional notation, it inculcates good habits from the outset and it provides you with a very broad base of technique that is applicable to all styles of guitar.
https://www.amazon.com/Modern-Method-Guitar-Volumes-Complete...