So you have a Ubuntu machine and a Mac OS X machine. You decide you would love to use an application on your Ubuntu machine while on your Mac. No problem!

install SSH on Ubuntu;

sudo apt-get install ssh

Now install x11 on your Macintosh. Should be on your OS X cd or you can search Apple's site.

Fire up x11, which you will find in /Applications/Utilities/

Select xTerm from the x11 menu and type:

sudo ssh -Y  linux-username@your-linux-ipaddr app_to_run

 Here's pySol running from my Ubuntu 7.10 laptop on my wife's G5 Tiger machine. 

If you have problems you may need to clear the known_hosts file;

sudo pico .ssh/known_hosts 

Have fun!