It is FREE, but looks like limited for 2 user to be able to connect, link:
...A complete solution for remote access to your Linux or Solaris workstation. It allows 2 users to connect at the same time no matter what their location is, and share the desktop.
NX Small Business Server - allow up to 10 sessions.
Install ssh server (without this you will failed to connect to 22 port):
sudo apt-get install ssh
Install node and server packages:
Adjust server to work with Unity:
restart NX server:
sudo /usr/NX/bin/nxserver --restart
Install on Windows client version of nx-clinet:
Select Desktop = "Unix / Gnome" to connect to Ubuntu.
Login - is user name from target PC
Attention: to access you existing session (like in VNC), select Select Desktop = "Shadow" and during login you will get list of sessions to join, select first session where Session name = "Local display" and Dispaly="0". Row with Dispaly="1001" will open new session.