A review tells me that the only viable choices now are...
OpenWeatherMap (which is what I've implemented)
https://www.weatherbit.io/
https://www.apixu.com/
https://darksky.net/dev
https://www.5dayweather.org/
http://www.myweather2.com/
https://developer.yahoo.com/weather/ -- now needs oAuth, which seems excessive, and I don't trust Yahoo not to completely drop the API completely soon
https://www.metaweather.com/ -- someone's side project, so not sensible to integrate
A review tells me that the only viable choices now are...
OpenWeatherMap (which is what I've implemented)
https://www.weatherbit.io/
https://www.apixu.com/
https://darksky.net/dev
https://www.5dayweather.org/
http://www.myweather2.com/
https://developer.yahoo.com/weather/ -- now needs oAuth, which seems excessive, and I don't trust Yahoo not to completely drop the API completely soon
https://www.metaweather.com/ -- someone's side project, so not sensible to integrate