I have a Macbook Pro and a Windows laptop. I can attach both of them to an external monitor.
Ideally I would like some sort of network based screen sharing so I can use all three screens with my Mac while the external screen is still attached to Windows box.
This would allow me to take my Mac easily when I go for work and help me get to a dual (or triple) screen setup quickly when I come back. I can spend some money for this as well but I would be interested i open source option as first choice.