Christine Porter is hooked on the MyFitnessPal app. In October, after deciding to lose 50 pounds, Porter started typing in everything she eats, drinks and any exercise she gets. "This is my main page here," says Porter. "It's telling me I have about 1,200 calories remaining for the day. When I want to record something I just click the 'add to diary' button. I'm on it all day either through my phone or through the computer." She says she's lost 42 pounds in nine months. Health apps like MyFitnessPal are turning smartphones and tablets into exercise aids, blood pressure monitors and even devices that transmit an electrocardiogram. But the explosion of apps is way ahead of tests to determine which ones work.