Synergy

Synergy allows you to use one mouse and keyboard to control several computers - Windows, Mac, Linux, it doesn't matter. It does this spectacularly well. The one issue I have with it is that it does no encryption: solving that with ssh is described here. [As of 2014, Synergy includes something it refers to as "encryption:" I couldn't get it working at all, so I continue to use ssh.]

There's a graphical configuration utility. I usually edit the config file by hand. The simplest configuration with two screens side-by-side:

section: screens
    laptop:
    desktop:
end

section: links
    laptop:
        right = desktop
    desktop:
        left  = laptop
end

One machine is considered the "server": that's not selected in the config, it's chosen by running "synergys" on that machine - the machine on which you'll be using the mouse and keyboard (yes, my setup is a little odd, the laptop is the server). The other machine(s) is/are the clients. For a secure configuration, start the server as usual on one machine ("laptop", that also has an ssh server running on it): synergys . Then on the client run ssh -f -N -L localhost:24800:laptop:24800 laptop . This returns immediately, at which point you run synergyc localhost. You should now be done.

On the first attempt, you may want to run synergys -f and synergyc -f localhost so that the applications run in the foreground and it's easier to determine where you went wrong (if you went wrong).