!! General
* Synergy: control multiple machines with on set of mouse/keyboard
* X11: remote display/control
* VNC: remote display/control with lower abstraction level but high portability

!! Windows-related
Windows does not know about having multiple displays like X but there are solutions that create virtual displays that can be shared/exported:
* http://www.zoneos.com/zonescreen.htm
* Maxivista (not free)
 Windows has remote desktop but a special license is required to allow multiple users simulaneously.