Donut™ lets you record loops in two dimensions.
A detailed explanation of how the app works is available here: http://thestrangeagency.com/circles-within-circles/
You can use it like a phrase looper, by recording horizontally while using the step controls to step through vertical recording position. You can use it as a realtime processor by using ThruRecord mode. You can record and playback at completely differently speeds. You can even record backwards! You can record and playback diagonally across the Donut™ surface, creating glitchy staccato sounds. You can record and playback with multiple heads, recording or playing multiple positions at once. You can spread the heads vertically to access different portions of the Donut™, or you can spread them horizontally to create an echo effect.
Imagine two dimensional time!