The two big X Windows windows managers for Linux are Gnome (what I usually use) and KDE. However, if you want to try KDE on top of Mac OS X, you can try out...

KDE on Mac OS X

The stable version is 3.5. The brave among you can try the 4.0 development release. Me? I'll just change my Linux X window manager when I want to use a different interface. The Mac OS X GUI is just fine for me. :-)