I've found Synergy extremely useful where I have two PCs and five monitors with different monitors dedicated to different PCs. Instead of remembering to press the KVM keys to switch the keyboard and mouse over, I just move the mouse and as it moves off the edge of one PC/monitor the mouse and keyboard automatically switch over to the other PC/monitor. The only caveat is that its configuration is very non-intuitive.
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.