Do you have a bunch of people sneaking around that you want to not only catch, but expose to the world? If so, then this laser tripwire project is worth a try. You have to hook up a laser pointer, a photoresistor, and a webcam all together, and voila, you have a laser tripwire that will capture a photo of the intruder and upload a link to the photo on Twitter.
Think of it as a basic home security system. In fact it would be great if more security systems used this while the home-owner is away. The cops and Twitter would have a picture of any perks who break in.