Synergy is an Open Source app that allows you to control multiple machines from a single keyboard/mouse. Unlike VNC, PCAnywhere and friends the idea is not to control remote machines but for when you have multiple machines on your desk.
At work I have my workstation and my laptop, with Synergy I can move my mouse to the bottom of my monitor and it jumps to my laptop. The keyboard then sends input to the laptop.
This is all done over a simple TCP connection, which is fine until you start typing in passwords. After a bit of tinkering I have a script to run the Synergy client over an SSH connection using local port forwarding.
Using an SSH agent is highly recommended for Windows (try Putty's paegent).
You'll obviously need an SSH server running on the machine with the Synergy server.