The iShred LIVE app is one of the first amplifier/FX simulators I became aware of when I first discovered it was possible to play guitar through the iPhone. It was also through this app that I discovered the GuitarConnect Cable (the first guitar/iPhone interface I purchased). I mention that because of the interfaces I currently own, only GuitarConnect Cable and AmpKit LINK work with iShred LIVE. GuitarJack does not.
One of the things I still like about this app, is that it’s free. You need to purchase the effects that you require from inside the app, but it comes with enough for free to keep anybody playing for hours. Like I’ve done for a day.