This program uses VNC to create a dual-monitor effect. So if you have two 
or more computers on your desk and you are tired of having several keyboards 
and mice around to control them, this is the program for you. While running 
this program, you can move the mouse pointer beyond the right or left edge 
of your primary display and the pointer will appear on the other computer 
screen. If you have ever used x2x or a computer with two graphics cards, you 
get the idea.

It's open-source and available for both Windows and Unix.

Windows: http://fredrik.hubbe.net/win2vnc.html
Unix: http://fredrik.hubbe.net/x2vnc.html