Apple Flavored Unix: X Servers for Mac OS

MI/X
- The only free X server for Mac OS
Mac X
- Apple's own X server for Mac OS, this server can run rooted, that is, with a "root window" behind the X client windows, or "rootless", in which every X client runs in a separate Mac-style window.
eXodus
- White Pine's X server for Mac OS, this also supports rooted or rootless operation.
XTen
- Tenon's X server for Mac OS, this server also offers a unix runtime environment, and as a result, XTen supports running local X clients, XTen is also quite fast, as it runs full screen, and can bypass QuickDraw and handle the display on its own.

MachTen also comes with an X server, but that's overkill if all one needs is an X server.