Not exactly guitar oriented, but I get the feeling that a good proportion of Guitar Moderne readers are interested in synthesizers—and the Moog Model 15 Modular iOS App looks like a great one.  Also, it is controllable by external controllers, which could easily include the MIDI Guitar app. Mostly, I just love the meta-ness of vintage modular technology from the Sixties being replaced by keyboard synths in the Seventies, regaining popularity in hardware form in the last couple of years, and now being available as the most modern of technologies: an app.