ClipTen free calculators — plain URLs, free, no key, no sign-up.

Currency to words (live exchange rates, updated daily):
  https://clipten.com/calc/currency-to-words?amount=USD+3+million&to=INR
  https://clipten.com/calc/currency-to-words?amount=2.5+crore&from=INR&to=USD,EUR,JPY
  amount: a number, with optional currency and unit: 3m, 3 million, 2.5 crore, 12 lakh, 1.2bn, 500k, ₹10 lakh
  from: currency code if not in amount (default USD). to: one to five codes, comma separated.

Number to words (international and Indian systems):
  https://clipten.com/calc/number-to-words?n=25000000
  https://clipten.com/calc/number-to-words?n=3.5+million

Exchange rates against USD, JSON:
  https://clipten.com/calc/rates

Add &format=json to any of these for JSON. Full docs: https://clipten.com/calc. The same tools with a page: https://clipten.com/tools