I recently heard about a really cool open source software program on Leo Laporte's radio show "The Tech Guy". It's called Synergy. The software allows you to share the keyboard and mouse of one computer with other computers, and it does this all through software, no hardware needed. The typical way of doing this in the past was by installing a hardware KVM switch that you physically hooked your keyboard and mouse up to (and video, if you want), and then had to either physically press a button on the switch, or press a hot-key on the keyboard to have your keyboard and mouse switch to the other computer.
What's really cool about Synergy is I can configure where the other computers are in relation to the server machine. When I move my mouse off the screen to the far right, synergy will begin to control the computer to the right.
For example, I work with three different computers on my desk. I have my PowerMac G5 as my main computer in the center with 3 displays, a Mac Book Pro on the right side of the monitors for my G5, and an old Windows laptop on the left side of my G5 monitors. Synergy works flawlessly with a multi-display setup, so when a move my mouse off the right side of monitors on my G5, I know have control of my MBP, the keyboard works for which ever computer has the mouse. When I want to work off of my windows laptop, I just move the mouse over to the left four monitors, and now I'm controlling that PC. I'm now able to save a few seconds when I'm switching computers because now I don't have to reposition my hands when I switch computers.
Also, on a Mac there is SynergyKM, which is a GUI install of the software.