I recently had to install GIMP on my MacBook Pro, and the step to install X11 temporarily stumped me. So I though I would share a little OSX gotcha that will help you install X11 if you should ever need to.
X11 is a UNIX windowing system that allow developers to create graphical user interfaces on the UNIX operating system. It also allows UNIX users to view these GUI’s. As OSX is UNIX based, X11 allows your Mac to run the thousands of applications that were created to use X11.
The steps to install X11 on OSX are pretty simple. Insert your OSX Install Disk #1. A finder window will open that will show you all the installers and folders on the disk. Here is the gotcha though - to install X11, you have to double-click on the Optional Installs package. However, due to the organization of the icons in the finder window, you must scroll down to find the Optional Install icon. I swear I missed this at least 3 times while trying to install X11.
Hope this tip saves you some time!