What's the most useful smartphone app, real or imagined?

Darcy Ahl, who was in the car when her 16-year-old son tried to answer his phone while driving, has created an app that parents of teen drivers might really like -- as might the rest of us on the road with folks who have more manual dexterity than common sense.

Reports The Los Angeles Times:

"Ahl came up with an idea for a smart phone application that prevents drivers from talking on the phone and texting while driving. … The application iZUP uses GPS technology to detect a car's speed. If a car is traveling over 5 miles per hour, the application sends incoming calls to voice mail and holds text messages. It also blocks outgoing text messages and calls, except emergency 911 calls."

For now, the application is available only for Research In Motion (NASDAQ:RIMM) BlackBerry phones and handsets that run on Microsoft (NASDAQ:MSFT) Windows Mobile and Google (NASDAQ:GOOG) Android operating software.  

What do you think would be the most useful app to have on your teenager's phone, whether he or she would appreciate it or not? How could other software for Apple's (NASDAQ:AAPL) iPhone, Palm's (NASDAQ:PALM) Pre, and other smartphones help out worried parents?

Sound off in the comments box below. We're listening even if your teen isn't. For each post, the Fool will donate $0.10 to the Thurgood Marshall Academy in Washington, D.C., in furtherance of financial literacy.

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Fool online editor Kris Eddy owns no shares of any stocks mentioned in this article. Try any of our investing newsletters free for 30 days. The Fool has a disclosure policy.