synergy

At my day job I work between two main computers and sometimes three. So far, I have found great success running Synergy to enable using a single mouse and keyboard between them.
With my PC set up as a server and my Macs set up as clients, I have basically configured the desktops of my computers to connect in one giant row of desktop space. The effect is so seamless that I often feel frustrated that I can't drag and drop between them.

A couple of items in the popular ordered list format:

1. Go PC to Mac. Seriously. It just works better that way.
2. On the Mac, use Synergy KM. Synergy KM is the version that uses a graphical preferences pane. Just because you can use the Command Line doesn't mean it makes sense.
3. In deference to the first point, configuring the screen locations for the server on Windows is unnecessarily complicated. Don't forget to configure your way back onto a Desktop once you've configured your way off.
4. On the Mac, the server is supposed to find clients via bonjour, but I can only get it to work by manually entering the IP of the server. This is annoying when your network doles out IP addresses dynamically.
5. The only time I've ever seen Bonjour work was when I swapped out a PC with a co-worker who then suddenly found that she kept having to fight with a phantom (Me) for control of her mouse. She had no idea what was going on and I laughed and laughed at her.
6. You'll still find yourself needing a mouse/keyboard on each machine at some points. I find it easiest to just keep the keyboard tucked under the monitor or shelf that the computer sits on.
7. By default, the client and server will dole out "special" keys as if you were using the default keyboard layout for that operating system. You can configure this, but why? I put little stickers on my keyboards to help keep track of what key is what key in what mode.
8. The processing of your mouse movements, however, is somewhat processor intensive-- and having a laggy mouse can really suck. My G5 iMac is more prone to this lag. In fact, I've never noticed it on my Dual-Core MacBook.

A fun fact: I worked for a CAD-CAM software company for a number of years whose flagship software product was called "Synergy." Back then, I lobbied against the name because it was an abstract noun that required too much thinking. I still feel that way, even about this 'OS independent input device sharing' software. Synergy is supposed to evoke the idea that by working together, two independent things are greater than the sum of their parts, right? But that's just stupid in the context of sharing a single keyboard and mouse across multiple computers. That shouldn't require synergy... that's just how shit should work.