I just discovered this great novelty site Riffstation Beta which seems to be free. I don't play guitar, keyboard or uke - but this auto-play along site makes me want to pick up an instrument and have a go.

It visually represents the chords as a youtube video plays in real time. However, my query as a non-musician who has placed some recordings on youtube is this... What is it picking out to represent as the chords they show (the overall melody, cadences, backing instruments, forefront vocals)?
They are vague in how it works, and as a non-muso I don't know how accurate it is (they claim at least 85%, but then that's 15% of duff chords!)

Is it musically useful for learning, (it can even show chords in left handed variation), or a flash in the pan toy that stops you from learning true playing. Of course these are just general fair comment opinions being sought... not legally derogatory comments on the software or its makers!