Synergy2
From Applepedia
Synergy2 is a really cool client/server network application that allows you to use the same keyboard and mouse across multiple monitors attached to different systems. For example, say you have Mac and a PC that you want to use at the same time. Synergy2 is the application to do it. You hook your keyboard and mouse up to one of the systems, that becomes your server. On the server you configure how you monitors are setup, and other tweak options. You also specify a client access list. Then, presumably through SSH, you start the client service on the other machine which connects to the server. You can now pass your mouse from one screen to the other. Whichever monitor your mouse is on, is the one that you type on. It's as seamless as a typical multi-monitor setup on one system.
