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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.
So you don't want to put serious time or any money and you want quality content. Hmm ... At least you could be more specific what you want to learn. There are free books to learn Bitcoin, for example - https://github.com/bitcoinbook/bitcoinbook and it's a good book, so read it ... or not.
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.