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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.
In my opinion, Enpass has the best balance of privacy, open-source, offline-first, etc. It's essentially a proprietary UI over a sqlite/SQLCipher database (fully open source CLI available: https://github.com/hazcod/enpass-cli).
It can - but needn't - be sync'd any number of ways (iCloud, Dropbox, Google Drive, OneDrive, WebDAV), if that's what you like. Because it has WebDAV sync support, you can use it with ownCloud or NextCloud without much fuss, and not have to trust a third party at all.
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.