Weather Underground was the first online weather service, but until now it didn't have an iPhone-specific app. That will all change soon when the Weather Underground app arrives on the App Store.
The developers of the app provided TUAW with a pre-release version of the software, which is expected to be approved by Apple shortly. We'll update this post with links to the application on the App Store as soon as it's available.
The free app takes advantage of official weather stations as well as the huge network of over 23,000 personal weather stations around the world. What this means is that users have the ability to get current weather readings for much closer locations. For example, there are four National Weather Service stations in the Denver area, none of which are closer than about 8 miles from my office. The Weather Underground app provides a list of stations that are closer -- two of them are about a mile away.
The Weather Underground's website displays weather information on an interactive weather map called WunderMap®. The iPhone app includes WunderMap and a number of overlays that can be toggled on or off, including animated radar, satellite, webcams, and severe weather updates.