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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.
I'm also using EteSync with the etesync-dav "bridge" for synchronising calendars and contacts (even though, that happens outside of ProtonMail; for me it was more important to have the sync working with Thunderbird and Android devices.
NOTE:
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Hence, a higher number means a more popular project.