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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.
There are also things like the IRC Highway that allow you to search for books and download them using IRC (https://github.com/evan-buss/openbooks is a friendly UI for using this). You can also check subreddits like r/opendirectories which might have books in PDF format etc.
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.