X11 is the mother of many things, one being remote computing. Since X11 is still the graphical engine on most modern Linux distributions these now old-school X11 functionalities are still usable.
Just for fun, I set things up on Ubuntu and OS X Mavericks, without using SSH. This is important since SSH has the -X option for tunneling X11, which makes a X11 client server configuration very simple. And thus you can find guides to do this all over the internet. X11 works best on a LAN and SSH tunneling is not needed when connecting over a LAN. Also SSH tunneling kills your performance due to encryption used. Some “guides” solve this problem by configuring the “none” encryption method. The real solution is not using SSH tunneling at all.
I love plain old X11: run the server on your client and set $display on your server for your X11 clients. When running into authentication problems, you might need to configure a MIT-Magic-Cookie…