Deep Synth is universal iOS app priced $4.99. It is now very capable with midi keyboard support and AUV3 (Audio Unit extension support). It is unique synthesizer that can generate many sounds by changing the waveform of the oscillator. There are 5 points attached to the wave that can be moved to create diferent peaks. Also very interesting to understand and hear what affects what in the sound.
You can play it via MIDI keyboard or using the touch screen and can also use it as an instrument in GarageBand as it is AUV3 compatible.
You can hear some sounds here: