Robotic Drums is a clearly laid-out Drum Synth with a Probability Sequencer for iPad and iPhone. It has Audiobus, Midi clock and Dropbox support.
More info at http://roboticdrums.com
Sound Cells is an interactive, generative music app capable of creating its own sounds or generating midi to drive other external or iOS synths. Sound Cells also supports Audiobus for inter-app audio communication.
Many experiments happen here, currently heavily focused on iOS Audio App development.