Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.
ReSlice - Slice your audio with an advanced, rhythmic arpeggiator and FX.
Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.
Omnisphere - Piano, pad and synth VST for DAW's.
AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹