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Mattermost
Mattermost is an open source platform for secure collaboration across the entire software development lifecycle..
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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.
Another way opensource but paywall projects are implemented is by publishing everything publically but then requiring licence keys that are hard to circumvent (looking at you mattermost)
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.