For those of you Ninjas who are used to using multiple machines and monitors and having to shell out hard-earned moolah for a KVM Switch there is a free as in beer open-source software alternative called Synergy
Synergy can run on multiple Operating systems including Windows/Linux and Mac, But can be a bit tricky to configure, but not to worry you can download another open source wrapper tool that utilizes Synergy and makes configuration a snap: QuickSynergy
Once Synergy/QuickSynergy is installed and configured on the machine that is the host of the keyboard and mouse and also on all the client machines that need to share it, you can now seamlessly use the same keyboard and mouse to navigate across screens and operating systems. And if that was not enough it even enables cut and paste across operating systems..!
So go ahead and give QuickSynergy a try and put your old KVM switch for sale on eBay.
P.S You might want to follow the steps mentioned here too, to avoid problems with elevated prompts in Windows Vista and above.
