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fosscord
Discontinued 📬 Spacebar is a free open source selfhostable discord compatible communication platform [Moved to: https://github.com/spacebarchat/spacebarchat]
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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.
Matrix is the only real alternative that's fully open source.
There's Fosscord, which is supposed to be a server and client compatible with Discord. Not sure how far along they are at this point though.
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.