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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 is also an addon called Straight Reward that will increase the ease of your cards if you repeatedly get them right, it helps keep your cards from stacking up too much especially if they're ones you consistently get right.
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.