Running X11 - 5. Other X11 Possibilities
VNC is a network-capable graphics display system similar in design to X11. However, it works at a lower level, making implementation easier. With the Xvnc server and a Mac OS X display client, it is possible to run X11 applications with Mac OS X. Fink provides X11-based VNC packages for some platforms. Check the entries here
WeirdX is an X11 server written in Java. It also supports rootless mode. Sources and a java jar file are available at the web site.