From the Synergy website: “Synergy lets you easily share a single mouse and keyboard between multiple computers with different operating systems, each with its own display, without special hardware. It’s intended for users with multiple computers on their desk since each system uses its own monitor(s).”

I use my laptop as the “server” and then have a linux machine in my office that I can control with the laptop keyboard and mouse…. just by moving my mouse over to the right edge of my laptop screen until it jumps to the linux machine.
At home I use the same software to control a MacOSX machine from my Windows Server 2003 primary machine.

This product is simlar to a KVM, but allows both machines to have their own monitors and for you to see/control those machines at the same time (without switching between). One side benefit is that you can copy-n-paste between machines (simply copy some text in machine one, mouse over to the other machine and paste), show me a KVM that can do that!

You can download MacOSX, Linux and Windows binaries at:
http://synergy2.sourceforge.net/