x2x

x2x creates a 1 pixel wide invisible window down one side of your desktop. When the mouse pointer enters this window, x2x begins sending X mouse movement instructions to a second X server.

Hence, you have the illusion of being able to move your mouse pointer off one screen and onto another. x2x also handles things like cut & paste and keyboard focus, so you can truly work effectively with two X displays and one keyboard and mouse.