You can use x2vnc or win2vnc to control the mouse and keyboard of another computer.
x2vnc - allows control of a vnc server from an X-windows computer
win2vnc - allows control of a vnc server from a Microsoft Windows computer
They can be configured to "attach" the displays edges, so that it seems like the desktops are connected.
For example, controlling an X-Windows from MS Windows:
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