Brazil RESTful zip codes API.
ViaCep API is a free Geocoding API that requires no API key, is served over HTTPS and has unconfirmed CORS support.
Webservice gratuito para consulta de endereço via CEP, suporta Ajax e retorno nos formatos JSON ou XML.
Details from viacep.com.br.

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.
The ViaCep API is provided by ViaCEP (viacep.com.br). Webservice gratuito para consulta de endereço via CEP, suporta Ajax e retorno nos formatos JSON ou XML.
Yes. The ViaCep API is listed in the Geocoding 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 ViaCep API requires no authentication, so you can start making requests without signing up.
CORS support for the ViaCep API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The ViaCep API is served over HTTPS, so it's safe to use in production.