[samatman]

Clang leaps to mind. Most of LLVM was developed under Apple's auspices, although the project predates Apple hiring Chris Latner. Swift is also open source, although in practice it's almost always used in a macOS context. Which I think is a shame, it's a great language for writing the sorts of servers for which people usually reach for Go.

That's not a complete list of programs Apple has open-sourced, although clearly open-source software isn't what they're best known for. But clang alone is a worthy contribution which deserves to be recognized.

[stock_toaster]

FoundationDB was open sourced a few years after they bought it too.