ssh to your unix server, passing the -X option:
ssh -X -l user@server
When you start an X Windows app from the ssh session, it’s output will be forwarded back to your Mac.
More details here.
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This does not work on Mac OS X Mountain Lion. Any suggestions?
Do you have XQuartz installed? Mountain Lion no longer comes with X11 libraries, you have to install them separately. See (http://support.apple.com/kb/HT5293), and for XQuartz, see here (http://xquartz.macosforge.org/trac).