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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.
As others have said, Cognito is an option with Amazon. Another option is Auth0 (free for basic use, low cost for some other features depending on your needs. Authentication is free and secure.) https://auth0.com/
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.