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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.
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AppFlowy
AppFlowy is an open-source alternative to Notion. You are in charge of your data and customizations. Built with Flutter and Rust.
I'm browsing this list, but it is hard to tell which one is still maintained.
I would suggest you take a look at harpy. It's a Twitter client and still seems to be developed (last release was about a month ago).
https://github.com/AppFlowy-IO/AppFlowy for the lazy people.
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.