A KVM (Keyboard-Video-Mouse) is a box that allows you to control 2 computers with one monitor, keyboard, and mouse. It switches from one PC to another by a combination of key (default 2 times "scroll stop." But it is editable) or by pressing the button box.
I know these appliances have already used professionally, this one does its job perfectly, you plug in 2min and it requires no external power or driver installation on PC. By cons, you need a USB keyboard and mouse, PS2 / USB adapter does not.