Off the top of my head, clang, launchd, libdispatch, Bonjour (Rendezvous) and the ARM64 backend for LLVM.

I would never claim that Apple is a fervent open source company, but here are a bunch of their open source projects:

http://www.macosforge.org/

Clang comes to mind. You can see all their opensource stuffs on the Apple website

https://opensource.apple.com/

Clang, CoreFoundation (or rather, the CFLite subset of it), the xnu kernel, libdispatch/GCD, their Objective-C runtime, launchd, libauto (Objective-C garbage collector), mDNS/Rendezvous/Bonjour/Zeroconf, stand out as examples.

Apple is definitely not afraid of closed source when it suits them (no source code available for most system APIs or bundled apps) but they also have a fairly extensive open source portfolio.