Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.
cassini - Polyphonic Synthesizer for iPhone/iPod touch
AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹
zMors Modular - Full featured modular synthesizer for iOS
Ripple Maker - A coffee machine that turns selfies & photos into latte art