TouchOSC allows to remote control and receive feedback from software and hardware that implement the OSC or MIDI protocols such as Apple Logic Pro/Express, Ableton Live, Renoise, Pure Data, Max/MSP/Jitter, Max for Live, OSCulator, VDMX, Resolume Avenue/Arena, Modul8, Plogue Bidule, NI Traktor, NI Reaktor, Quartz Composer, Supercollider, vvvv, Derivative TouchDesigner, Isadora and many others.
Lemur allows you to turn your device into a fully customizable MIDI control surface. Lemur has more features which makes it a lot more powerful than TouchOSC.
However, this functionality comes at the cost of complexity and makes Lemur harder to get started with. But if you want to make sure you have every single control option available, it is the most powerful control software you can have.
iMS-20 is a complete music production app for iPad. It consists of a complete recreation of the legendary Korg MS-20 analog synth with the patching capability, plus a 16-step analog sequencer based on the Korg SQ-10, a six-part drum machine which can create drum parts by simply tapping the buttons, and a seven-channel mixer with 14 types of effects.
In addition, iMS-20 offers dual Kaoss Pads allowing you to generate music by simply stroking the pad and moving your fingers over it.