Microsoft Windows

There are several open source packages for X11 available for Windows. In the Spring of 2011, COS 333 had students use Xming. 

Mac OS

From Apple's Knowledge Base HT5293

X11 is not included with Mountain Lion, but X11 server and client libraries for OS X Mountain Lion are available from the XQuartz project: http://xquartz.macosforge.org. You should use XQuartz version 2.7.2 or later.