"Hidden is an app that claims to help you track down and find your Mac -- it runs in the background of your desktop computer, completely invisible, and only starts to send out location information when you actually go to the Hidden website and choose to flip it on. That way, if your Mac is ever stolen, you can log on from any computer and immediately get updates on where your stolen computer is, helping you track it down as quickly as possible. Getting it back is up to you, but just finding a stolen or lost computer is often all you need to do to provide proof that it's yours anyway."
And now this normally $20.00 USD program is free until January 1st, 2011. All you have to do is go to their website, register a user name and password, and download the program. Simple as that. And since it follows it's name, there is no trace of the program on your computer once you install it and restart your computer. It doesn't show up in the applications folder and there is no preference pane associated with it. If your laptop is stolen, all you do is log in at the Hidden website, mark the laptop as "stolen" and wait for the tracking to start. Their FAQ page is pretty thorough if you have questions. All in all, not bad for free.
UPDATE: Apparently they had such a great response they have ended this promotion as of today. I suppose it is within their rights to do so, but it certainly doesn't do much for goodwill when all of the printed material you put out says January and you end it a week early.