I'm inclined to yawn at the prospect of yet another weather service/app, but Forecast is making me giddy. It's a new offering from The Dark Sky Company, makers of the eponymous app that I rely upon to get 'hyperlocal' weather (i.e. to-the-minute notifications that it's about to rain over my house).
Like the Dark Sky app, Forecast is smooth, attractive, and a pleasure to use. It differs in that it builds and expands upon Dark Sky in profound ways: it promises seven-day global forecasts; offers historical weather conditions; delivers even slicker fluid animations; and adds multiple layers of weather information. There's also an API for developers. You have to check it out for yourself.
Forecast demonstrates just how polished and pleasant a web app can be. Add it to your home screen on your iOS device, and you'll swear it's a native app that you downloaded from the App Store.
I currently use Dark Sky and Garmin's My-Cast to get my weather on my iOS devices. On the Mac, I often geek out with WeatherSpark (which offers an amazing depth of information, but is lamentably Flash-based). Forecast may displace all of these services.