Check out Synergy. If you have 2 network-connected computers, say a desktop and a laptop, you can share the keyboard and mouse of the desktop and go back and forth between monitors.
Really impressive, no more KVM switches.
It works similar to the multiple-monitor setup, with the addition of a workable-keyboard and mouse every time you move back and forth between monitors.
What does this mean?
I use my personal PC at home, and my laptop for development & work. I could be surfing the web on my desktop while developing on my laptop, and then be able to test web applications on my desktop while running/debugging on my laptop.
Synergy User Guide
Plus it's free!
One neat feature that would be really impressive is if you could drag an icon from one computer desktop to another (eg. copy from one desktop folder to the other just by dragging and dropping). Now I think we're talking about Mac simplicity here.