With a little effort you can create all sorts of useful devices based around an Arduino, and one great example is the following motion alarm by Instructables member milcarlsandover. After identifying a need for a simple baggage alarm - that is, when the bag is moved an alarm sounds - a neat solution was designed and built.
Motion is detected via an accelerometer - a neat choice as this allows your alarm to be aware of false positive movements caused by vibration, and once a suitable movement has been detected - the alarm sounds. It can then be deactivated by entering a sequence of presseses on a small keypad. A demonstration is shown in the following video:
For complete details on this project and a review of how to make PCBs at home, click here to visit the alarm's Instructable page. And for more, we're on facebook, twitter and Google+, so follow us for news and product updates as well.
If you're looking to recreate such a project, a good start would be with the AM3X accelerometer module:
This tiny 3-axis accelerometer module can operate in either +/-1.5 g or +/-6 g ranges, giving your project the ability to tell which way is up. Ideal for robotics projects, tilt sensors, vehicle data loggers, and whatever else you can dream up. For more information and to order, click here.