Synergy: Share Keyboard and MouseSynergy (http://synergy2.sourceforge.net/index.html). I should say it's one of the most easy-to-use software I have ever encountered. Here are the quick steps (you can read more on their user's guide).
1. Install Synergy on both machines.
2. Setup one machine as the server.
3. Add the machines left, right, top, and bottom of the server (if any).
4. Start synergy as client on those other machines.
That's it and it's working. Only thing is that all machines should be in the same network.
One cool thing is that clipboards of all machines are shared. So copy from one machine and paste in the other is possible. Also it enables you to activate screen saver on all machines at once.