I was looking at the other products that the developer of JollysFastVNC works on, and I saw that he is developing ScreenRecycler. This looks like a cool product, that lets you use a second computer as a additional display.
This is different than synergy, where you can use the same keyboard and mouse to control two different computers, but with synergy, when the cursor is on the remote computer, you are accessing that computer.
It looks like ScreenRecycler is implemented as a VNC server that tricks Mac OS X to see a second display. Any thing that is supposed to be displayed on that "virtual" display is sent to any attached VNC clients.
This is a pretty cool product, especially when the mac that you are running doesn't have any available video ports.