Popular weather data site Weather Underground (Wunderground) has a new version of its Weather Underground API and two things are notable: its former XML service is being phased out in favor of JSON. And it has a very clear pricing structure, though there's a free version for non-commercial projects.
The API pricing structure is clearly defined and quite reasonable. It fits into the freemium category with zero cost plans available for developers and other light users. To get serious, like 10 thousand calls per minute kind of serious, only costs between $300 - 750. I have to think that if I was able to generate that many API calls per minute to Wunderground’s service, 750 bucks would be a bargain. The real kicker is the historical data set which will cost you $500 - 5,000, though that, too, is free in the developer version. There's also a basic paid plan for $20 per month, with a reasonable rate limit of 5,000 calls per day.
A key note here is that Wunderground decided to offer the JSON format first in this new incarnation of their API. XML was the format of choice in the original iteration. This is yet another sign of JSON’s wide acceptance as a standard in the API industry.