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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.
If you want to host yourself https://matrix.org/
Rocket.Chat (Open source, UI is quite similar to Discord) Element (Matrix based) Tox (Security focused alternative)
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.