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exchangerate.host
Exchange rates API is a simple and lightweight free service for current and historical foreign exchange rates & crypto exchange rates.
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
If anybody is looking for a free alternative that just works, I'm using [1] for a demo on my product's homepage [2]. I suppose it is a bit different in scope though (updated daily instead of near realtime).
[1]: https://exchangerate.host/
NOTE:
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Hence, a higher number means a more popular project.