This is for a help desk scenario where you need to see the desktop of a remote user.

Install x11vnc on remote machine. Then on the remote system (as root) run x11vnc –localhost
Now if the machine is inside a private network we may need to hop with SSH

user@mysystem:~$ ssh -L5900:localhost:5900 external-host
user@external-host:~$  ssh -L5900:localhost:5900 internal-host
user@internal-host:~$ sudo su -
user@internal-host:~#apt-get install x11vnc
user@internal-host:~# x11vnc --localhost

Now start your local vncviewer and connect to localhost. xvnc4viewer works great on an Ubuntu system.

Ref:
https://help.ubuntu.com/community/VNC