Did you know that the popular glass plate known as the iPad can be used as a musical instrument? This classic piece of 'clean' Apple iDesign, covered in grease and who knows what else, can rock!
The video's on his Youtube channel and the posts on his blog show that Rob has an extremely musical mind as well as serious software engineering skills. His apps are actually playable and useful for 'real musicians' and.. hold on to your hats.. he's into microtonality as well!
iPad musical instruments typically use the multitouch glass plate feature for either a continuous 'sliding' control or a 'gridded' kind of control (like a Korg Kaossilator). Rob Fielding has combined these two approaches into one, more versatile design, including xenharmonic properties like 're-fretting' an instrument to a different tuning.
There are other useful apps for musicians, but mostly educational. For regular harmonic theory (chords, scales) the Octavian app is ok.