I have three monitors and two computers: one laptop, one desktop. The desktop is powering one of the monitors, and the laptop is powering the other two.
I used to use a simple USB KVM switch to move keyboard and mouse control between the computers, but then, almost randomly, I came across Synergy. It seamlessly connects two computers such that you can share a keyboard and mouse without a KVM switch.
The experience is fantastic. My two computers behave like they were one – as I move the mouse to the left of my middle monitor, the mouse and keyboard hop to the other computer as if they were the same machine. Better still, I’m using two different operating systems (windows on the laptop and fedora core 7 on the desktop).
It’s a little tricky to set up but there are tutorials on the web to lead you through it. I highly recommend this method of sharing a keyboard and monitor with two computers.