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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.
Signal wrote their own open source protocol, libsignal, to provide end to end encryption; or as they put it "a ratcheting forward secrecy protocol that works in synchronous and asynchronous messaging environments." Source I'd wager good money a chunk of that space is for said library and dependencies PLUS the regular rigamarole for an Android app.
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.