Apple guides development for WebKit and releases code for mDNSResponder (Avahi is admittedly used more in OSS operating systems, although NetBSD-current notably uses mDNSResponder), Darwin Streaming Server (the backend to their proprietary QuickTime Streaming Server, and ALAC.
They don't release everything as open source, but they certainly don't disregard its importance.
Webkit is mostly LGPL. This is a bad example, because the codebase is derived from KDE.
mDNSResponder is Apache. This was released to drive industry acceptance of Bonjour/Zeroconf.
Darwin Streaming Server is APSL (the Darwin license) and patent encumbered. IIRC, there was talk of it being open sourced to drive H264 acceptance.
Darwin mostly does count as sharing code for the sake of being open, and to their credit Apple does keep updating it, but I struggle to think of other Apple releases for which this appears to be the most important consideration.
Apple guides development for WebKit and releases code for mDNSResponder (Avahi is admittedly used more in OSS operating systems, although NetBSD-current notably uses mDNSResponder), Darwin Streaming Server (the backend to their proprietary QuickTime Streaming Server, and ALAC.
They don't release everything as open source, but they certainly don't disregard its importance.