Other than checking the temperature or the forecast, I often use the radar function of weather apps. I like seeing radar in motion, so I can guesstimate what's going to happen in the future. In the video below, I tried the radar experience in four apps: The Weather Channel, Weatherbug, Yahoo Weather and My Radar Pro. The only app you'll pay for up front is My Radar Pro at US$1.99, and I feel it's worth it.
Let me know your favorite weather radar app in the comments.