Ubuntu 12.04 and XDMCP

XDMCP is a really neat protocol. You can use an X server to remotely connect to a linux system running it and then you can use it as if you are sitting at the system. I use it on a couple of my Linux server computers and it is incredibly easy to setup on Ubuntu 12.04.

The recent versions of Ubuntu use LightDM for the display manager. You can run a bare X windows session, but there's no easy way to start programs. That's why Window Managers like Gnome, KDE, icewm and others exist. Then you have Display Managers like xdm, gdm, lightdm and others that will give you a graphic login and the ability to switch which Window Manager you are using with a GUI.

http://askubuntu.com/questions/66431/how-do-i-configure-lightdm-to-work-with-xdmcp

To configure LightDM to enable XDMCP do this:

Edit /etc/lightdm/lightdm.conf
add

[XDMCPServer]
enabled=true

That's it!

You can log in using X on another linux system, or using an X server on Windows such as Xming. I highly recommend Xming when on Windows.

Want to run the system headless? That's where you remotely access the GUI without having a local copy running.

http://lists.freedesktop.org/archives/lightdm/2012-February/000233.html

Edit /etc/lightdm/lightdm.conf
then add

[LightDM]
start-default-seat=false

Then sudo restart lightdm to load the new settings.
You'll have to login again if you were using the GUI but you will see that the server side GUI didn't start up, while you can still log in over the network.