[nemothekid]

I'm pretty sure Darwin has been open source since forever - at least since 2003 according to Wiki. This is the first time I've seen it on GitHub rather than Apple's internal open source microsite (https://opensource.apple.com/)

[xenadu02]

This is correct. xnu/Darwin, libDispatch, the Objective-C runtime, and many other components have been on opensource.apple.com for years.