Fullscreenweather works just like Google Maps, meaning you can use your mouse wheel to zoom in and out, as well as use it to drag around the surface. It also includes quick toggles to change the terrain type, as well as what weather layer you're looking at--like temperature, precipitation, and cloud cover.
The company says it's been designed especially for touch-screen devices, including the Apple's upcoming iPad.
One of my favorite features is that the service creates a shortened URL for each location, so that you can share the map with someone else with all the same settings you've enabled. It's a nice touch, especially compared to Google's own map links, which are well over 200 characters long.
The service is completely free--and ad-free--although clicking on things like detailed weather reports will take you to WeatherUnderground.com, where there are ads.
Of course, Weatherunderground is not the first company to combine big, mouse-friendly maps with weather reports. We've also seen some impressive efforts from the Weather Channel (which also used Microsoft's maps at one point), Accuweather, and Google itself.