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
webkit is a fork from KDE, so "initiated" is doing a lot of heavy lifting there.
llvm also existed from before although apple supported the development.
I was curious if apple had actually started any FOSS project.
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