Either you have a monitor that has dual inputs, which is the case with me (Sony GDM-FW900), or you buy a KVM switch and use one monitor, keyboard and mouse for two or more computers. The KVM switch is a godsend for many people, but as far as I'm aware it limits the video quality as well as the resolution. I'm pretty sure that there are splitters or switches that allow you to simply split the one existing cable going from your monitor between two computers...this should be much like a little box attached to a pair of cables that allow for two connections and the switching of them...Seems logical but I'm not aware of any such thing.
As for KVM switches, I know that Belkin makes good ones, and I also know that many larger Sony monitors have dual inputs.