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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.
MELPA is definitely the biggest package archive, probably mostly on account of the lower barrier to entry compared to GNU ELPA (maybe the difference would have been less pronounced if NonGNU ELPA had been there from the beginning, but one can only speculate). MELPA has its own requirements for packages, but indeed copyright assignment to the FSF is not one of them.
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.