WretchUp was developed by German duo Mouse on Mars as a bespoke live performance delay effect. Previously the subject of an Indiegogo campaign, the app recently reached its target and is now available on the iPhone and iPod touch.
It's based on 'analogue delay equipment', and the emphasis is placed firmly on mashing and twisting the input sounds with the onboard Delay Time, Filter and (double) Feedback controls. Basic looping functions are available to dial in, and you can choose an external input, of course.
As it's a live performance effect primarily, WretchUp isn't currently flush with AudioBus support, but this'll be coming in a future update.