Ah, Synergy, what a wonderful tool. I use it at work to connect my Debian box to my Windows XP box. The Debian box has 2 monitors, so I have three monitors on my desk.
It's a lot of fun to mess with people as they walk by. Grab a maximised window on one monitor, move it to the other monitor, then start typing in a window on the XP box.
Throw in
xmove, and you can really blow Windows people's minds, as you "throw" applications from one X11 system to another X11 system across the network. Someday, I'd like to set up an X11 server on my Windows box, and then whip up a way to "drag" a window from the Debian box to the Windows box.