Right now only the ECB and blockchain.info APIs are used which are totally free and don't require an API key at all. In the past there was a strong requirement on this because data was fetched in client. But now the data is aggregated by a batch job run on the backend (rinkcalc.app), so any API could be used, as long as I could afford to pay for it (some of them are outrageously expensive...)
Some decent options are below. A lot of the ones I found were simply flipping the ECB data behind a pay wall so it's important to find ones that actually use multiple data sources and will be robust to future problems like the ECB deleting a currency from their API (as happened with the Ruble).
Right now only the ECB and blockchain.info APIs are used which are totally free and don't require an API key at all. In the past there was a strong requirement on this because data was fetched in client. But now the data is aggregated by a batch job run on the backend (rinkcalc.app), so any API could be used, as long as I could afford to pay for it (some of them are outrageously expensive...)
Some decent options are below. A lot of the ones I found were simply flipping the ECB data behind a pay wall so it's important to find ones that actually use multiple data sources and will be robust to future problems like the ECB deleting a currency from their API (as happened with the Ruble).