Here is my latest blog post on a method of using a laptop equipped with an X11 server as a KVM or graphical dumb terminal (not to be confused with “kernel virtual machine”) to display an entire desktop environment on one of the virtual terminals of the laptop. You might be familiar with running an X11-compatible program over SSH, seeing a remote GUI app displayed on your local machine. You could also run a whole desktop environment such as “xfce4-session” over SSH and attach it to a virtual terminal (which you switch between using Ctrl-Alt-F1 through Ctrl-Alt-F7, usually, on Debian-based systems).
This is nice if you have a few always-on Raspberry Pis laying around, and you usually use SSH to remote login and control them, but you would like to have a full desktop environment, not just one app, show up on your local computer.
I should look into it, but I don’t know whether or not X11 provides for any kind of video compression when updating regions of the screen. I only ever use this over a an 802.11g LAN so even if there is no video compression I don’t notice any lag. I haven’t tried using it over the Internet, but I am guessing it would probably be a bit slow.