Synergy

I just set up Synergy so that I can use my WinXP laptop keyboard and mouse to slide over onto my multi-head setup at work and control those machines.

I was going to use x2x (which works under cygwin), but it seems to only see the right hand head. A bit of Googling turned up Synergy and, aside from a little bit of a learning curve, it works great.

The trick is to run the server process on the machine that has the physical keyboard and mouse, and run client processes on the machines that you want to incorporate into your virtual multi-monitor setup.