I have a lot of computers at home. I have 3 of them in my office and almost always have at least 2 of them on. It would be truly convenient to use all of these computers at one time as a "super computer". All of this is possible with Synergy.
Synergy enables you to arrange your monitors side by side (and stacked on top of one another) and treat you monitors like one big desktop. Each of these monitors is run by a seperate machine that can be running different OS's. They can be run from one computer mouse and keyboard. This is superconvenient for programming. I can run documentation on one machine while developing on another. I can copy and paste between Windows machines and Linux machines. I can use one keyboard to drive all my machines. In other words, I'm in heaven.
If you have multiple machines and long to make them useful try Synergy.