Audiobus: Use your music apps together.
What is Audiobus? — Audiobus is an award-winning music app for iPhone and iPad which lets you use your other music apps together. Chain effects on your favourite synth, run the output of apps or Audio Units into an app like GarageBand or Loopy, or select a different audio interface output for each app. Route MIDI between apps — drive a synth from a MIDI sequencer, or add an arpeggiator to your MIDI keyboard — or sync with your external MIDI gear. And control your entire setup from a MIDI controller.
Download on the App StoreAudiobus is the app that makes the rest of your setup better.
Synthmate by Sohla is free
Nice synth and control surface from the makers of Bip
real-time control, low latency
multi-touch interface, change anything while playing
5 note polyphony
1 oscillator unit per touch: control pitch, volume
1 lfo unit per touch: control speed, amount
1 delay unit : control time, feedback
Y axis assignable control
edit scales, range, portamento and micro-tune
record and playback performances
load/save all settings
audiobus as input.
CoreMIDI support : use synthmate as a midi controller
https://itunes.apple.com/au/app/synthmate/id403402387?mt=8
Edit: sorry, already posted. Beaten to it by @NoiseHorse