Ever since ROLI released the Seaboard RISE back in September, there's been more interest in expanding the expression of your basic keyboard device. The Seaboard range, and particularly the more affordable RISE, offers MPE (Multi-dimensional Polyphonic Expression) thats note on, key position along the length of it, and pressure (all per note) for the keyboard surface. Not that they are the first, but they certainly appear to be the most successful.
Apple meantime, with the iPhone 6S also introduced 3D Touch to its capabilities. No doubt the boffins at ROLI had a light bulb moment, as since then, they've been working to bring thier new free NOISE app to the market. Essentially on iPhone 6S, NOISE gives you the MPE playing experience (aside from the largess of the actual keyboard) but on your phone screen. You can apply the same pressure and positional control that you can with the Seaboard together with its own Mobile Sound Engine.
This sound engine presumably takes elements of the EQUATOR software synth - also keenly tuned to work with MPE control data and gives you 25 sounds to play with - more are available via in-app purchases - hey someone's got to pay the bills.
So if you want to experience the idea of MPE but aren't ready for a Seaboard, this ones a no brainer. Currently only for iPhone (iOS 9 up), iPad version is not (yet) available.
Available free via the iTunes Store