iWeather (free, ad-supported with in-app options) is a graphically interesting and reasonably complete weather app. The problem for most new weather apps like iWeather is being creative enough to get people to stop using some of the leading weather apps like Yahoo Weather, Apple's built-in app, and other popular apps like WeatherBug.
iWeather has a very clean look, and is gesture-driven. Pull down to add cities, tap for details, pinch for multi-city display. The app includes a 5-day forecast, and automatically figures out your current location using GPS. Each city displays temperature, humidity, wind speed, wind chill, and visibility. Background colors change based on time of day and the weather conditions. You can also share weather information via Facebook, Twitter and email. It's a worldwide app, rather than being focused only on US cities.
I like having four cities tiled to see them all at once, something I haven't seen on too many other weather apps. The paid version lets you display more cities and eliminates the ads, which are really the big problem with iWeather -- they are placed at the top of the screen where they are sure to distract you. Worse, the ads are animated, so your eye is drawn toward them. The US$0.99 in-app purchase fixes that, but iWeather has to compete with Yahoo and Apple's free apps that provide even more information. WeatherBug has a free version with ads, but they are at the bottom of the display and, to my eyes, less distracting.