MAC: X11 on Mac 10.5.6

The X11 download on apple.com is for Panther 'only'. So it will never be installed successfully. You will get the error prompt like "You cannot install X11 on this volume. A newer version of this software already exists on this volume."

Your best bet for staying current with bugfixes is to install the latest X11 package (at the moment X11 2.3.0) released by the Xquartz project, which contains Xquartz-1.4.2-apple5, and includes many fixes outside of Xquartz. Afterwards, just install the latest binary versions of Xquartz released periodically as fixes are being made. To see the complete list of changes in release X11-2.3.0, please go here.

After this installation, run the x11.app from /Application/Utilities/, then run terminal from the X11 Application menu. In this xterm based terminal you can run those x11 applications installed by macports or fink, also run those like gimp which need x11 environment.

For example:
1) run gedit
gedit
2) run gimp
open -a gimp or open /Applications/Gimp.app