WebKit is itself open source, so you can't discount that just because it was derived from another open source project.

Aside from LLVM/clang and WebKit (both of which are significant enough to deserve massive accolades all by themselves), you also have stuff like launchd (which didn't catch on outside Apple, but hardly their fault), libdispatch, and mDNS.

But apart from the sanitation, the medicine, education, wine, public order, irrigation, roads, the fresh-water system, and public health, what have the Romans ever done for us?