Synergy. About Synergy (from synergy2.sourceforge.net):

Synergy lets you easily share a single mouse and keyboard between multiple computers with different operating systems, each with its own display, without special hardware. It’s intended for users with multiple computers on their desk since each system uses its own monitor(s).

Redirecting the mouse and keyboard is as simple as moving the mouse off the edge of your screen. Synergy also merges the clipboards of all the systems into one, allowing cut-and-paste between systems. Furthermore, it synchronizes screen savers so they all start and stop together and, if screen locking is enabled, only one screen requires a password to unlock them all. Learn more about how it works.

Synergy is open source and released under the GNU Public License (GPL).

I am Loving Synergy – I just set it up to share my iMac keyboard and mouse with my work laptop, and it was super simple.

My setup before Synergy involved a 22″ monitor and docking station with its own keyboard and mouse sitting next to my 24″ iMac, with its own mouse and keyboard.

I downloaded the appropriate Synerygy files to each system, planning to run the iMac as the Synergy server. I had a few issues at first: I set up the iMac as the server system, but the Windows laptop would not find it. Since my work system is on a domain and doesn’t talk happily across my home network, I made a simple addition to my hosts file on the Windows machine, allowing it to find the iMac by hostname directly to it’s local IP, and then everything just came together.

The Synergy documentation is clear, and the software itself is very simple.

I highly recommend it to anyone trying to use separate systems on the same desk without a KVM. Free, simple, and works just like it says it will.