Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.
Dexed - Dexed is a multi-platform, multi-format plugin synth that is closely modeled on the Yamaha DX7.
AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
FL Studio Mobile - Create & save multi-track music projects on a smartphone
ZynAddSubFX - ZynAddSubFX is an open source software synthesizer for Linux, Mac OS X and Microsoft Windows.