Synergy is an open sourced software that lets you easily share a single mouse and keyboard between multiple computers with different operating systems, each with its own display, without special hardware.
With synergy, all the computers on your desktop form a single virtual screen. You use the mouse and keyboard of only one of the computers while you use all of the monitors on all of the computers. You tell synergy how many screens you have and their positions relative to one another. Synergy then detects when the mouse moves off the edge of a screen and jumps it instantly to the neighboring screen. The keyboard works normally on each screen; input goes to whichever screen has the cursor.
You can arrange screens side-by-side, above and below one another, or any combination. You can even have a screen jump to the opposite edge of itself. Synergy also understands multiple screens attached to the same computer.
You can even cut/copy text, html or an image on one screen and then switch to another screen and paste it. It works well even if the two computers are using different Operating Systems.