Synergy lets multiple machines share a single mouse and keyboard, and makes switching between them as easy as moving the mouse from one screen to another on a multi-monitor setup. Just move the mouse off the edge of the screen, and it enters the screen of another machine, taking the keyboard with it. It even manages the clipboard, so you can copy a piece of text on one screen and paste it into a window on another machine. It’s open-source and supports Windows, Linux and the Mac.
The config program could be more intuitive, but once it’s set up it works really nifty. It’s great for working on my laptop and desktop at work, or for switching between my Mac and my Ubuntu machine at home. Which is why I hereby give Synergy the official “recommended by Martin” award.