The OpenWeatherMap PHP SDK by Christopher Norton allows developers to integrate the OpenWeatherMap APIs into their PHP applications. So far, only the OpenWeatherMap Current Weather Data API is provided, but others will be added in the future.
Metwit is a crowdsourced API platform that provides third party applications the ability to integrate media-rich local weather and environmental data that is hyperlocal and in real-time. The Metwit API Platform collects weather information from Twitter, Instagram, NOAA and other sources which makes it possible for third party applications to use tweets, photos, and metatags to provide users accurate and interactive local weather information.
Though Google Maps may still be the choice of most developers, Bing continues to be a contender. Microsoft evangelist Chris Pendleton points out a new Weather.com feature and mentions it has used the service since back when it was called Virtual Earth (the switch only happened this June).