So Gaga has an app for her record (apparently it's terrible)
Bjork's was meant to be quite good (did anyone fork out for it?)
Gwilym Gold has an app that I was blown away by last year http://drownedinsound.com/releases/17250/reviews/4145520
Metronomy are the latest act to wander into the app world, but doesn't sound like it's all that
What music-related apps have you used and liked? Doesn't have to be artist related. Just wanted to see what apps I should be giving a go that are vaguely to do with music.