Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.
Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.
cassini - Polyphonic Synthesizer for iPhone/iPod touch
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
zMors Modular - Full featured modular synthesizer for iOS
Vital - Vital is a spectral warping wavetable synthesizer with drag'n'drop modulation workflow and animated preview of the synth's inner workings where needed. Comes with many modulation sources (including audio-rate), MPE support and FX chain.