I have been waiting for someone to do this: use a cheap motion detector and an old cell phone to act as a low tech LoJack car theft detector. It’s a really simple concept: when the car starts rolling, it calls (or sends a short text message to) you, the police, or whoever you tell it to. There are lots of ways to make a system like this expensive, but this is impressively low rent: BLADOX Turbo Motion mobile phone accessory
The next step would be to attach the phone to a GPS receiver. That way the phone could tell you exactly where it is as the perp speeds away. Put that together with some realtime mapping software, and you’ve got a scene almost straight out of Goldfinger.