EchoVNC is a full-featured version of VNC based on UltraVNC
with echoWare integration. 

With EchoVNC on both sides of a connection, two VNC users 
can connect to each other regardless of firewall or router 
configuration. VNC becomes "firewall friendly" -- no more 
port-forwarding!

EchoVNC uses the same EchoWare software that Kaboodle
uses. It is free, Windows-only, fully compatible with all flavors 
of VNC, and of course open-source. You can find it here:

            http://echovnc.sf.net

Note: EchoVNC is a product of Echogent Systems, Inc., which 
      hosts this VNC FAQ-O-Matic website.