Free and open source synth AK Synth One (coming soon).

[Cinebient]

Looks nice so far:
http://audiokitpro.com/audiokit-synth-one/

[analogmatt]

Hopefully, it becomes a great music making tool for everyone!

+ Unlimited Poly Synthesizer
+ Over 100+ Presets crafted by famous sound designers (from M83 & Kanye West to Red Sky Lullaby)
+ Five Oscillators (2 Morph, FM, Sub, Noise)
+ AudioBus & Inter-app Audio
+ Ableton Link
+ Classic poly arpeggiator
+ AUv3 Plug-in support to use in your iOS host
+ MIDI in (Control with a MIDI Keyboard or DAW)
+ Vintage-Style Step Sequencer
+ Individually Morphing Oscillators
+ FM Oscillator w/ Mod
+ Dedicated Sine/Square -12/24 Sub Osc
+ 4-Pole Vintage Low-Pass Filter
+ High-Pass/Band Pass Filters
+ Mono glide
+ Beautiful Sean Costello Reverb
+ Assignable LFO w/ 4 waveforms
+ Multi-tap (ping-pong) delay
+ Touchable ADSR Envelopes for Amp & Filter
+ Detune Oscillators by Semitone & Hz
+ Full Source code included
+ Preset Import/Export & More…

[Wormhelmet]

Awww, I thought it was the ipad version of Synth One by Ichiro Toda.

Been hoping for that one for awhile.

[AnX]

+ Over 100+ Presets crafted by famous sound designers (from M83 & Kanye West to Red Sky Lullaby)