Noise.io is an interesting-looking synthesizer app for iPhone, due to appear in the App Store in the coming weeks.
What's interesting about it is the emphasis on live performance. This is an app designed for use on stage, and comes complete with the advice: "Simply connect the sound output of your iPhone/iPod touch to the mic/line
input of your mixer, recorder or sound card - and there you are, now you can enrich your tracks or mixes with superb sound effects."
Right now, we shall just have to take the developers' word for that, because the app isn't officially released and all we have to go on are the screenshots and a single preview video that hints at what's to come. For seven bucks, it looks like it might be hard to resist.