I would recommend getting a KVM switch. That way you don't have to have 2 keyboards and 2 mice to control 2 computers. If you really don't need it, there ARE just monitor switches, but they're less common.
There are two kinds.
1) mechanical
These types just have a mechanical switch that flips between the two monitors. I think you can find them for $15 if you look around. They tend to cause image degredation beacuse of unshielded wiring. Maybe there are some good ones out there, but I've never seen one. Some monitors won't switch refresh rates after being toggled like that and it can mess up the image and potentially (in rare cases) damage a monitor. Also, if you're switching a mouse, it's a good idea NOT to use these because mechanically switching a PS2 port can do damage to the equipment and will sometimes cause loss of sync with the mouse, usually requiring a system reboot.
2) electronic
These types maintain sync with the monitors and this is what most good KVM switches are. They keep the mouse powered and don't screw things up. They often let you switch computers through the use of a keyboard 'hot key' instead of a physical switch (like 'ctrl-alt-f1', 'ctrl-alt-f2', etc). Some even have software to do it and provide onscreen displays. These range from $50 to $25000.
PS - the $25000 model is an Avocent 8-user digital KVM-over-IP (With 50 software licenses) that let you login and remotely control your stuff from over the Internet. Neeto.
Eric