Synergy is an open-source dingus that allows you to share a single mouse and keyboard across multiple computers and operating systems. Think of it like a soft-KVM switch. I've been playing around with it for the last few weeks since I received my new MacBook Air in tandem with my Windows 7 work desktop.
What I Like
Shared Clipboard. Copy some text, a link, etc from Mac, paste into Windows. Works great, provided I can remember not to confuse the keyboard shortcuts.
Single Mouse & Keyboard. I try to keep a clean desk. Multiple mice (or Magic Trackpad) and keyboards is a little too cluttered for my liking unless it's a necessity.
What I Don't Like
Config is a pain in the ass. There's multiple forks of Synergy floating around on sourceforge, google code, among others. Some work with each other, some don't. Some have a decent GUI for selecting where each device is and can change keys on the keyboard to emulate the corresponding ones on an OS (think ⌘ key.) Some are easier to use than others. It means trial and error and time wasted.
Connectivity is spotty. At random occasions the link between the server and client machines stops. Seriously disruptive when you're used to moving the cursor over to the other OS/screen and you slam into the edge of the screen instead and bang the cursor against the screen edge in frustration (not that I would ever do such a thing.) Then you get to play 'figure out which connection failed, server or client' which means opening up that closed MBA and restarting Synergy on it (since it doesn't have a keyboard or mouse connected anymore) then repeating the same on the Windows machine. Annoying.
At the moment, I'm actually typing this up with a standard Apple Wireless Keyboard and Magic Trackpad, as I've discovered that the client machine can't detect some of the nice features of the server machine's devices. So, while I was able to get the trackpad working in Windows 7 (mostly) none of it's features worked when the cursor was on the OSX screen.
I'm planning to try swapping the server-client roles for the Windows 7 and OSX machines, respectively, to see if it helps at all.
If you can tolerate a slightly messier desk, the MBA & external monitor & wireless keyboard & trackpad make a great combo.
If you've got multiple machines/OSes and only enough desk space for a single mouse & keyboard, and you can forgive some of Synergy's quirks, it's a nice little bit of software.