teleport, a smart software KVM
abyssoft: teleport -- "teleport is a simple utility to let you use one single mouse and keyboard to control several of your Macs. Simply reach the edge of your screen, and your mouse teleports to your other Mac! The pasteboard can even be synchronized between the computers."
I have been talking to friends about doing exactly this software since 2001 (maybe even earlier), but obviously didn't get motivated enough to do it. I wanted it to work across platforms, mind you, not just Macs, but Windows machines as well, and why not *nix too? This is something that should be straightforward, but probably is quite tricky. It could probably re-use a lot of VNC code... but really, it's much simpler than VNC.