At the heart of Mac OS X one finds Darwin, an open source core that integrates a diverse collection of powerful technologies in a robust, flexible architecture. Darwin is like Linux with a day job: By day, it stays discreetly in the background, running Mac OS X. By night, Darwin shows its open source roots: hackable, extensible, and the product of the same community, culture, and traditions that created Apache, sendmail, GNU/Linux, Mozilla, and UNIX itself.
Users are happy, but engineers also get to have their fun.
That’s what it’s all about really, isn’t it?