You may already know all about Weather HD -- the app made a nice splash when it was released early in the iPad's life. But Vimov has just released it for the iPhone, and even on the smaller screen, it leaves quite an impression. The idea is that instead of a tiny little icon or a big number telling you the weather, you get a full, bright screen of whatever the weather's like near you (or what it's going to be like in an hourly or daily forecast).
The app is super simple on purpose, so if you're looking for full maps, video, or more detailed information, you'll have to go elsewhere (I usually use the free Weather Channel app for more in-depth weather updates). Still, for just 99 cents, Weather HD is worth having around if all you need to check every once in a while is the temperature and forecast.
And heck, if you already bought it for the iPad, the new version is universal, so you've got iPhone and iPod touch support anyway.