Free weather API with forecast data similar to Dark Sky.
Pirate Weather API is a free Weather API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
Details from pirateweather.readthedocs.io.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Data from Personal Weather Stations called senseBoxes
The Pirate Weather API is provided by Pirate Weather (pirateweather.readthedocs.io).
Yes. The Pirate Weather API is listed in the Weather category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The Pirate Weather API requires no authentication, so you can start making requests without signing up.
Yes. The Pirate Weather API supports CORS, so front-end JavaScript can call it directly.
Yes. The Pirate Weather API is served over HTTPS, so it's safe to use in production.