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
http://drownedinsound.com/in_depth/4147167-dis-does-singles-18-11-13--metronomy-lily-allen-disclosure-summer-camp

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.