Synergy

Find yourself with too many keyboards and mice on your desk? Synergy is an open-source tool that can help. Synergy links multiple computers (even with different operating systems) to one keyboard and mouse. To switch monitors, you simply direct your cursor off the side of one screen and onto the other. Synergy works across the systems via TCP/IP networking, so there’s no special KVM hardware to buy or install. Synergy will also allow you to cut and paste between the systems, and it synchronizes screen savers and screen locking to kick in on all your computers at the same time.

Oh, and if your systems are already dual-monitor, then Synergy will turn you into a quad-monitor computing fool!
Synergy

Latest Version: 1.3.1
Platform: Microsoft Windows (most versions), Mac (OS X 10.2 or higher), and Unix X Windows.
Author: Chris Schoeneman and the Synergy team
Price: Free (FOSS)
Download Link: synergy2.sourceforge.net