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MangoDB
Discontinued A truly Open Source MongoDB alternative [Moved to: https://github.com/FerretDB/FerretDB]
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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.
Actually... fresh from the tree: https://www.mangodb.io/
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.