Shortly after my settling in at Apple, I became the Technical Lead for the Appearance project. The full-blown Appearance Manager actually gives the Mac switchable themes, where users can choose what type of look they want for their computer. This look affects all the visible user interface elements. Very cool stuff. This means more choice of software titles, and happier customers.
Happier customers mean happier developers, which means they develop more.
I'm a firm believer in giving developers every piece of information I can. Appearance is a good example, there are virtually no hidden routines, etc. Everything's layed out in front of them, so they can do whatever magic it is that they feel compelled to do. After the huge release of Mac OS 8, we worked on the Allegro project, or Mac OS 8.5. We did a lot more cool stuff in that release, with even more cool functionality that developers can use.