A lot of people think of Apple as super proprietary, but they've actually created AND open sourced quite a bit (recapping a bit from other contributors):
ZeroConf (Bonjour) - the thing that makes auto-discovery of printers & other computers work
mDNS - a core technology required for ZeroConf
WebKit - used in Safari, Chrome, etc.
clang/llvm - Apple's been a major driver of this compiler technology which offers significant performance possibilities as well as code analysis that was not really practical/possible before without 3rd party tools.
GCD (Grand Central Dispatch) - That takes most of the irritating problems out of threading and intelligently handles multi-core processors.
I'm sure there's plenty more that I'm not aware of. But note that each of these are critical tools that solve non-trivial problems and Apple had little-to-no business reason to Open Soruce these tools.
They also use standard formats/protocols where applicable... They make heavy use of XML or JSON where they make sense, ASN.1 for receipt validation, etc.
To Todd's comment:
Objective-C was actually OpenSourced by NeXT. And while Cocoa may not be open-sourced, Foundation Kit and Application Kit are.
CoreFoundation was created by Apple and is Open Sourced. It's not Objective-C code, but does represent a comprehensive set of utility functions that can be useful in many C projects.