Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
zMors Modular - Full featured modular synthesizer for iOS
Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.
ReSlice - Slice your audio with an advanced, rhythmic arpeggiator and FX.
AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹
cassini - Polyphonic Synthesizer for iPhone/iPod touch