Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹
FL Studio Mobile - Create & save multi-track music projects on a smartphone
GarageBand - GarageBand is a fully equipped music creation studio for Mac
cassini - Polyphonic Synthesizer for iPhone/iPod touch