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.