Swift comes to mind, and this website has more https://opensource.apple.com/projects/ (it lists WebKit, but I guess they count WebKit after it split from KHTML, I’d agree with you there)
Also, Bonjour originated at Apple and it is cross-platform, although Avahi probably is more popular
CUPS is associated with Apple as well, but it seems that it also did not originate there
I was going to say swift, but I figured someone would undercount that because it is so overly relevant as an iOS/Mac development language even though it really is a pretty good portable language. But man... undercounting webkit and clang+llvm is a new level of ignorance..
It is ignorance to call them apple projects when they really aren't. The copyright notice of webkit is full of emails like @kde.org… wonder why it's not entirely @apple.com?
The majority of the advancements done to make the standards compliant browser with great js performance were done at Apple. Going by the copyright notice as the the source of a vast majority of the dev in the 2000s seems pretty special.
Actually the arm64 backend is a grey area. I knew some guys that worked on it in secret at Apple. They definitely created and Developed it at Apple and dropped it on the community after the iPhone 5s launched, replacing the existing aarch64 backend.
I’m not 100% certain if they started with any bits of the previous aarch64 backend or not though.
Also, Bonjour originated at Apple and it is cross-platform, although Avahi probably is more popular
CUPS is associated with Apple as well, but it seems that it also did not originate there