Synergy right now, custom program later.
At the moment I'm using a program called Synergy (http://synergy2.sourceforge.net/) to allow me to use one keyboard/mouse for all 5 computers. The way synergy works is actually pretty simple. First, you setup one computer with a keyboard/mouse plugged in to run the Synergy server software. Then, you setup the other 4 computers to run the Synergy client software. You tell the client software to startup when the computer turns on and connect to the server machine (the one with the mouse/keyboard plugged in).
When you first start up you will have a mouse cursor on the server monitor(s) that behaves like normal. However, when you move to the edge of the screen the mouse will disappear off that monitor and appear on the opposite edge of the next monitor (you tell the server how the monitors are arranged so it knows where to go). It does this by intercepting the keystrokes and mouse moves/clicks and sending them over the network to the client machine, which then simulates them on that computer. In the end it behaves just as if you had 6 monitors connected to a single computer instead of the more standard 2-monitor setup.
At the moment, while waiting for Burning Crusade to be released, I have actually been working on writing a program of my own that does the same thing but design it more specificly for gaming. At the moment Synergy doesn't work especially well with WoW and it can be a bit of a pain to use at times. Also, I had to write a couple of hacks for the program to allow it to do keyboard broadcasting (discussed below) and hotkey fall through.
One of the main things that allows me to effectively multi-box WoW is the keyboard broadcasting feature I added into Synergy. The way it works is that when it's enabled (can be turned on/off with a hotkey) any keystrokes will actually go to all connected clients and the server instead of just to one machine. This allows me to, for example, press the number 1 on the keyboard and have all 5 computers behave as if they all received that keypress. I will go into more detail in future entries about how this allows me to multi-box WoW but without it, 5-boxing would be incredibly difficult.
My quest to level 5 World of Warcraft characters at the same time.