Googling the word "weather" inside any browser on your phone brings up a weather 'widget' on top of your normal search results and as soon as you set up your location (works with GPS location or manually) it'll show you a ton of information related to current and future weather. In their words:
At first glance, you’ll see content that we’d previously shown you before: current conditions and a forecast for the next few days. But by moving the slider over the next 12 hours, you can now see a detailed hour-by-hour breakdown of the changing weather conditions. As you do this, keep an eye on the temperature, wind speed and humidity and see how all these conditions are expected to trend across the day. You may also notice that the background color changes throughout the day. Of course, as you scroll further down you’ll see our regular web search results for your query.
Personally, I have limited utility for this, since I have a temperature indicator in my notification bar at all times, but it's a nice example of the types of mobile apps we could soon be enjoying thanks to HTML5 and it's many improvements and options to developers.
Image: Google Mobile blog