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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.
Silverbullet uses markdown, can be installed as a local app or as a website and has cool dynamic query features
I've had a good experience with Supernote. They do use a proprietary note format, but there are open source projects you can use to convert them to png, svg, or pdf. Supernote-tools works well and gets updated regularly.
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.