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#421
I apologize if it has already been recommended but it wasn't on the first page and didn't turn up in a quick search.
Input Director is a free Windows application I can't live without. Input Director lets you control multiple Windows systems using the keyboard/mouse attached to one master computer. Basically it is a software based KVM switch.
Both at work and my home office I have between 3-6 monitors (various quite a bit as I play with different systems), Input Director saves me 1- clutter of having multiple keyboards\mouse; 2- time having to switch between different hardware; 3- money; 4- resources on my primary system. I basically dedicate 1 system to my core "office" applications (e-mail, work IM, monitoring software). This keeps my development system clean of extra apps and fast but with Input Director it acts like it is just another monitor.
Synergy is an alternative that is cross-platform and also free but development was halted years ago and does not work well in Vista or higher without disabling too many things.