Last week my coworker I share office with went on his honey moon. As he didn't need his computer I figured I could use it to run browser, messenger and other bits of software that isn't needed on my main machine. So I had triple-monitor configuration but it wasn't very useful with two keyboards and mice.
After I found Synergy my workplace was heaven. Two machines, three monitors, one keyboard, one mouse.
Synergy is free (GPL) software that synchronizes mouse and keyboard one two or more machines over the network. The desktop on non-local machines acts like it was extra monitor on main machine. Well, you can't really drag windows there, but with a little organization that isn't a problem. Synergy also synchronizes clipboard so it can be really useful. Another great feature is that it's cross-platform software.
While playing with Linux & Mono on my second machine today, I plugged my secondary monitor in Linux box but I used only one mouse and keyboard. Very comfortable indeed.
I wrote about dual (and more) monitor configurations before. You may also remember MaxiVista. A very cool idea, but I never found it useful enough to actually use it.