long, long time ago, before smartphones were mainstream, we used to have PDAs (Personal Digital Assistant). you've probably never heard of it. </hipster>
jokes aside, I've been a passionate user of those prehistoric devices for many years. I owned a couple of iPAQs as well as the flagship Dell Axim X50v. I still have them in a drawer, gathering dust since years.
of course, I also wrote software for them (and on them, thanks to the wonderful Pocket C#). most of the stuff was never finished, and will never be. the only software that somehow survived is:
a virtual MIDI control surface
I think it was pretty advanced for the time (2008), but of course it's now vastly exceeded, in quantity and quality, by the current offerings for iOS and Android. still, if you are in a nostalgic mood, feel free to play around with it.