The ear is the most valuable and most neglected guitar skill — it's how you learn songs without tab, improvise in tune, and know what a chord will sound like before you play it. These trainers play a sound; you name it. A few minutes daily builds the skill faster than anything.
Two notes play in sequence. Name the distance between them. Start with the easy set and expand as you improve.
Each interval has a famous song that starts with it — hum the song to identify the interval. A few: minor 2nd → "Jaws" theme; major 2nd → "Happy Birthday" (first two notes); perfect 4th → "Here Comes the Bride"; perfect 5th → "Twinkle Twinkle" (first two notes); octave → "Somewhere Over the Rainbow."
A chord plays. Is it major (bright, happy), minor (sad, serious), or something more colorful? This is the fastest ear skill to develop and the most immediately useful.
Before answering, hum the notes you heard. Connecting your voice to the sound cements recognition far faster than guessing silently.
Five minutes daily beats an hour once a week. The ear improves through frequent short exposure, like learning a language.
Stay on the easy set until you're consistently right, then add intervals. Rushing to "all" just builds confusion.