I have absolutely no idea why I never saw this utility before! Instead of having a KVM switch sitting on your desktop, there's a piece of OpenSource software called Synergy2 that allows you the exact same functionality. And it's network based!
Simply, you install the software on all the hosts you want to share your mouse and keyboard on. Then you set up the "main" machine as the host and all the others as clients. In the UI, you establish the relationship between all the monitors. So for instance, here on my desk I have my laptop sitting to the left of me, then my main machine running Windows Vista on a nice flatpanel display, and to my right I have my Mac Mini. So I simply set up the following relationships on the server host:
laptop is to the left of vista
vista is to the right of laptop
vista is to the left of mac
mac is to the right of vista
Configuring it on the mac is a little trickier and involves editing a text based configuration file but if you follow the documentation it's really no big deal. So here I am, with three monitors, each connected to a machine, but only one mouse and keyboard. When I move my mouse pointer right on to the left edge of my screen, the pointer jumps over to the laptop's display. And moving the pointer back, it jumps back on to the main monitor. And if I continue moving right, I can go right across to my Mac. Copy-Paste even works between machines!
Get this really cool piece of software right here