eXodus from White Pine Software (603-886-9050) runs on any Mac with at least
1MB of memory and supports intermixing of X and Mac windows and also supports
the SHAPE extension. Version 5.0 became available 10/93.

Apple's MacX runs on MacPlus or newer machines with >= 2MB of memory and
system software 6.0.4 or later. Version 1.1 is fully X11R4-based.  It
supports full ICCCM-compatible cut and paste of text AND graphics between the
Macintosh and X11 worlds, the SHAPE extension (including SHAPEd windows on
the Macintosh desktop), an optional built-in ICCCM-compliant window manager,
X11R4 fonts and colors, a built-in BDF font compiler, and built-in standard
colormaps. Upgrades to MacX are available by ftp from aux.support.apple.com.
Info: 408-996-1010.

See NetManage with XoftWare for MacOS. (http://www.netmanage.com).

Tenon's MachTen (XTen?) X Window Software, Release 3.0, is a comprehensive X
display server and X client development environment. It includes an X11R5
server ported to MachTen/MacOS, standard window managers, an a set of X11R5
client-side libraries.  Info: Tenon Intersystems, 805-963-698, AppleLink:
TENON. See also http://www.tenon.com/

Also: 

Liken (1-800-245-UNIX or info@qualix.com) software enables monochrome 68000
Mac applications to run on a SPARC system running X.

Xport (1-800-245-UNIX (415-572-0200) or xport@qualix.com) enables Mac
applications to display on an X-based workstation by turning the Mac into an
X client. [Note: there are questions on whether this product is still
available.]

Intercon has a product called Planet-X which enables Mac applications to
display on an X server.

AGE Logic will ship XoftWare for Macintosh in March 1995.