I am always on the lookout for slick flash websites that work on my BlackBerry PlayBook. I was impressed when @BerryInformed pointed out WeatherSpark to us on Twitter. The WeatherSpark website provides tons of weather data for your location in their flash beta site. They definitely should package this up into a PlayBook app but it does work well in the browser.
It gives you radar maps, graphs, broken down forecast, historical weather, multiple sources of data and more. If you have been looking for such a think I highly recommend checking out www.weatherspark.com from your PlayBook! Its also fun to use on your desktop.