Tons of options. Search for VNC. If you want to forward your existing active X session you can use X11VNC, of you want to host sessions that do not require a user to be logged in, use a generic vnc server with tigervnc or tightvnc, or any of the other options for vnc server.
These can also be enabled at boot time so they are always on. But keep in mind you will need to open a port in the firewall so that they can be reached externally.