In 2008, Korg started a minor mobile music revolution with the DS-10 synth workstation for Nintendo DS, and now it's available – in considerably slicker, beefed-up form – for iPhone.
iDS-10 comprises two virtual analogue monosynths, a Voice synth and a drum machine, plus pattern (up to 64 steps) and song (up to 32 patterns) sequencing, and a Kaoss Pad that can be used for note input (with scale snapping) and real-time parameter control.
The analogue synths are based on the MS-10 and feature two oscillators, a multimode filter, an ADSR envelope, a handful of effects and a nifty patch-point modulation setup (albeit with only six targets).
The Voice synth lets you record or type in words for processing via a vocoder-esque engine – it's not intended for generating realistic voices – and the Drum Machine gives a surprising amount of control over its synthesised kick, snare, hats, tom and percussion sounds.
Although for serious music production on iPhone there are significantly more powerful options on the App Store costing less, iDS-10 sounds good, is a lot of fun to mess around with.