I recently bought the FitBit Aria scale. I love it. It tracks weight and body composition (though I'm skeptical of the accuracy of that function) in a web app.
My wife ordered us each a FitBit tracker last night (
http://www.fitbit.com/product/features ). I know several people who have, and love, them. They include a heart rate monitoring function, though I don't believe it will give real-time alerts.