I feel very behind the times. One of my favorite tools has not been maintained for years. BUT, there is a fork of it that is maintained: http://code.google.com/p/synergy-plus/
Even with a KVM, this additional tool is great. I have a dual-monitor setup where one monitor is on a KVM. The second monitor on my desktop system and my laptop screen are dedicated to those systems. With the KVM I can make either desktop bigger, but since I spend most of my time with my laptop (for work), Synergy+ lets me quickly drift over to my desktop and deal with anything displaying on the secondary screen (IRC, music, etc.).