He built his own version of the user interface framework, the software that sits right above the graphics processor on an iOS device. Apple creates that for developers — it was completely unnecessary for him to do this. But this is the kind of thing he considers “fun.”
“It’s insane,” he admitted. “But I wanted to experiment with different ways of driving graphics … Apple’s [UIKit] is the best, but I wanted to try.” The experiment was a resounding success — and now has a million guinea pigs testing his code via Letterpress. And, he said, “there have been zero issues” with what he built.

I'm a bit jealous.