Not a MacOS type UI, but I like WindowsMaker, which tried to emulate NeXTSTEP.
The whole UI was a lot more complex, and IMHO, probably the best-looking GUI ever designed. There's an effort to bring it back to life via GNUStep, and as a desktop environment, NEXTSpace:
WindowMaker tried to recreate the NeXTSTEP UI and succeeded admirably.
AfterStep might've started out trying to recreate the NeXTSTEP UI, but it went all kinds of crazy with colors and gradients. Since then its connection to NeXTSTEP has almost entirely been in name only.