In what can very loosely be classified as a free public service use for cell phones, a man by the name of Pete Tenereillo has developed a service called Trapster that lets drivers warn each other about speed traps by sending audio alerts instead of flashing their headlights.

According to NewsFactor, Tenereillo insists the system isn't designed to encourage dangerous driving. Instead, he says, the end result is that they slow down.

When users spot a speed trap, they report it to the Trapster database, which then sends an audio alert to other users who are approaching the spot. Trapster learns of users' locations by the car's GPS system or Wi-Fi. To make the service as useful as possible, Trapster has various systems in place to rate the credibility of user contributions.

The demo is for a Nokia smartphone, but Trapster works on about 10 different platforms.