Leaving your car lights on overnight is still one of the more reliable ways to kill the battery. Your Lights Are On! lets you notify other drivers if they've failed to switch off the lights via text message.
The no-frills and free service is pretty straightforward. If you see a car with its lights on, you send an SMS with the licence plate number to a dedicated number. Assuming the owner of the car has already registered with the free service (a simple process on the site), they'll get sent a message telling them to get the lights situation sorted pronto.
The service is only likely to get traction if a large number of people sign up for it, but it's not often you can try and be a good Samaritan for the cost of a text message. (The site proclaims that it will notify users by text if the free aspect ever changes.)
Picture by FelixR