Audulus - A universe of sound at your fingertips - Audulus is a modular music processing app
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
Blocs Wave - A free audio app for creating music loops.
Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.
FL Studio Mobile - Create & save multi-track music projects on a smartphone
AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹