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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.
The original game is completely open-source under the GNU license, it even has several spin-off's with very active communities. I reached out to the original creator for permission just to be 100% sure and he said its cool and was even interested in seeing the final product.
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.