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 Store
Audiobus is the app that makes the rest of your setup better.
Vember Surge is now free and soon to be available as individual VCV Rack modules
This isn't really new news is it was quietly announced a while back but the important bit is that Surge is now totally free and open source. The original developer is one of the founding partners behind the Bitwig DAW and if you're familiar with Bitwig you'll be at home in Surge.
As the review below points out. Surge is a bit of an ugly duckling in the UI department, but don't let that put you off as it has an incredibly powerful mix of wavetable and FM synthesis chops and a killer FX/Amp section that combine forces to killer effect.
As mentioned in the discussion title it's currently being modularised for VCV Rack but until then you can download versions for Windows, OS X or Linux.
And did I mention - it's free, free, free!