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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.
Materialize - mostly for streaming but they're starting to branch into warehousing, an excellent product https://materialize.com/
Preset.io is offering a hosted version of Apache Superset, completely open source BI (http://superset.apache.org/). Right now, Superset offers similar features to PowerBI or Looker but it's all open source.
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.