For iOS, but may also be available for Android and desktop…
Justin’s app is the best I’ve seen - by far! The combination of a solid video course, timed practice routines, and a deep catalog of curated songs calibrated for your current skill level that you can slow down and play along with is an unbeatable combination.
The only thing I would really like to see added to Justin’s app would be a looping feature.
For metronome and drum backing…
For slowing down, transcribing, and looping songs that aren’t in the Justin app, Anytune Pro.