Synergy is one of my most favorite tools, I just want to shout its coolness from every rooftop so that everybody knows about it
Synergy allows you to control the keyboard and mouse of any other computer on your network. It works sort of like monitor spanning does on the Mac, except when you move your mouse off the boundary of your screen it moves to the screen of the other computer. Using SynergyKM you can even copy and paste between computers!
Synergy/SynergyKM allows me to ditch the extra keyboard and mouse on those computers I don't use as my primary machine, and it is available for all platforms too, so not only do I control another Mac with it, but I also control my Ubuntu box - 4 monitors in total via my main laptop.
Synergy is the main open source component that drives everything, SynergyKM is the Mac system preference pane that makes setting it up very easy to do. I setup access to a new machine on the network in moments without needing to use the command line at all.
When most people I know discover Synergy they think it is quite cool, but I still run into people that haven't heard of it. If you are in this category, file this away in your memory banks, it's definitely very useful software - one of my favorite open source projects!