The Weather API

Wunderground provides a wonderful weather API.

They have a simple API request for all the different "feature" such as current conditions, forecast, history, hourly, etc..

http://api.wunderground.com/api/KEY/FEATURE/[FEATURE…]/q/QUERY.FORMAT

But it is not a really free API. Free use is limited to 500 calls a day, and 10 per min.