Me auto respondo para el que le pueda servir...
It is 100% Free.
No API key is required!
No cap on frequency of API calls.
Currency price is updated every two hours.
Support json format.
Very fast and reliable.
The api address is here. It returns 176 currencies prices together with country name, country code, currency name and currency price. The base currency is USD.
https://www.mycurrency.net/US.json