Synergy is a cross-platform, open-source replacement for a KVM switch. Instead of looping multiple computers through a piece of hardware, it lets you control them from a single keyboard and mouse. As a software solution, you might want to just go with the KVM switch. Synergy can be extremely difficult to configure, especially determining precise computer names and server locations. Even though the program lets you rename computers with simpler aliases, it can still be challenging to configure Synergy properly.
The interface is uninspired, but at least it's not confusing. Once you get Synergy running, you can switch from one computer to the next simply by push your mouse off-screen. You can configure that switching location to be on any or all of the four sides of your main monitor. Users can also configure screensavers to start simultaneously, and can link log-in information so that one hard drive doesn't hibernate while you're working on another one. The clipboards can also be linked, making it dead easy to transfer small chunks of information, such as URLs, from one system to the next.
This is a difficult program to set up properly, but it works well once going and might even be considered a must-have, or at least a must-try, for anybody who uses more than one computer at a time.