Apple Computer started putting motion sensors into their Macbook line of computers back in 2005; the feature was put into place to park the drive heads on a hard disk when the laptop undergoes a sudden jolt. It's a great way to make sure that your computer doesn't crash the heads if it's dropped, and that was, ultimately that.

Or so the folks at Cupertino thought. Because of how Mac OS X is built, the raw outputs from the motion sensor were fairly easy to find, and trial and error eventually let them be accessed in meaningful ways. (Shortly after this happened, Apple relented and just provided the internal documentation for the Macbook motion sensor). And by "meaningful", we mean "The data from the sensor was fed into all kinds of programs".

These macbook motion sensor apps run the gamut from productivity apps to games, to 'car alarm' setups and more. Many are whimsical, or very odd. Others just make you wonder why nobody thought of them before, because once you see them in action, you wonder why nobody ever tried using the Macbook motion sensor as an input device.

The first Macbook motion sensor app that actually improves productivity is SmackBook or SmackMac. This application works with a desktop manager utility (something that gives multiple desktops that can be toggled between via keystrokes or clicking on an icon) and the macbook motion sensor to let you tap your Macbook on the side of the computer to shift between virtual desktops. It's a pretty clever set of tricks, and has matured a lot since it was released in 2006. SmackMac and SmackBook Pro both work from the UniMotion application development kit, meaning you don't have to do strange things to the way OS X displays the desktop to install them anymore.

It should be noted that SmackBook's latest versions are the ones getting updated to the latest hardware; if you're using an older Macbook, or an iBook, you may have to roll back to an older version.

Another Macbook motion sensor app is iLevel. This is a desktop widget that displays like a carpenter's level, with a blob that will show how level (or less than level) the surface your Macbook is on. It's useful for figuring out how level your desk is, and is a lot of fun to use on an airplane. Word has it that some carpenters even use it on the job; after all, if your carpenter's level also lets you order more sheetrock from Home Depot, it's even more useful, right?

For the more scientifically inclined, there's an application put out by an Oregon seismologist called SeisMac, and SeisMaCalibrate. These gather information indicative of a seismic event, and will record the time and date of the event. By clever use of WiFi signals, it can even pinpoint the location of the computer and the direction the seismic event occurred in. Eventual enhancements for this include porting it to the iPhone, and letting people's Macs and cell phones provide a broad based seismology sensor network that reports in automatically for data gathering.

The last "productivity app' is MacAlarm. This lets you set your screen saver to a password protected one (standard functionality in OSX). However, it also makes your Mac set out an ear piercing shriek if someone picks it up when the app is installed. No word yet if anyone's worked out using an RFID keyfob to turn it off. (We've run into this one directly at trade shows; it tends to be less useful than it sounds).

Of course, motion sensors can be used for a lot more than just data gathering for seismology. They can also be used for games, as Nintendo has demonstrated amply, and the same for the motion sensors and accelerometers on the iPod Touch and the iPhone. This same logic has also been applied to the Mac, with versions of classic arcade games – and a few roller coaster sims – that use the motion sensor on the Macbook pro as an input device. Whether or not tilting your $1200 laptop to play video games is a compelling game experience is open to debate, but with the range of Macbook motion sensor games out there, the debate will probably be going on for some time.

The Macbook pro motion sensor was the first one widely documented (and the different versions of the Mac have slightly different motion sensors as Apple fine tunes the sensitivity). The ones in the iBook, and the standard Macbook are slightly different, and the new MacBook 17" apparently have a more sensitive motion sensor that puts its data out in a slightly different format.

If you're interested in developing your own Macbook motion sensor apps, there's an open source library called UniMotion that gathers all the data and puts it in a convenient format. There's extensive documentation on it, including which ranges of model numbers reverse the sign of the motion indicator – there's a set of Macbook Pros where the motion sensor is apparently configured backwards. It doesn't affect the head parking ability, but does make a lot of these apps a little odd. It's a cute weekend coding project…and who knows, maybe you'll find out something really clever to do with the data given. If you come up with a clever Macbook motion sensor app, be sure to let us know.