For those wanting a simple Linux alternative: If you have SSH access as the same user as the one running the X11 session on the remote host, you can do this:

    ssh -v -L 5900:localhost:5900 user@remotehost 'DISPLAY=":0" xterm -e "x11vnc -localhost"'

And then

    krdc vnc://localhost

or whatever VNC client you like.

For linux-to-linux this has replaced TeamViewer for me, together with zerotier.com for getting through NAT.