I know from personal experience that at least LLVM, Clang, Swift, and WebKit/JavaScriptCore have daily activity and also include non-Apple commit reviewers.
I also suspect CUPS remains pretty active. CUPS (Common Unix Printing System) started a long time ago and is used by pretty much all the Unix platforms. Apple bought the project in 2007. The license remains GPL2/LGPL2 and all the platforms still use it as far as I know.
The open sourcing of Swift was very beneficial. The project moved extremely fast the last years with lots of outside input and interest. I think this shows when OS works best: When it is not done out of obligation (or ideology), but because it can be useful as a tool.