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LibreOffice
Read-only LibreOffice core repo - no pull request (use gerrit instead https://gerrit.libreoffice.org/) - don't download zip, use https://dev-www.libreoffice.org/bundles/ instead (by LibreOffice)
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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.
LibreOffice! I've been using Microsoft Word and have recently switched to Libre to save some money.
Manuskript is an open source, free product that has many of the same features. The outliner allows you to jump around chapters easily, but it does not look anywhere near as elegant as Scrivener.
NOTE:
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Hence, a higher number means a more popular project.