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ankicommunity-sync-server
A personal Anki sync server (so you can sync against your own server rather than AnkiWeb)
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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.
Unfortunately, I reached this limit (with almost 1,8 million reviews and a couple hundred thousand cards), and Git Hub's alternative sync server seems not to work with IOS App. I'd willing to pay for more space, if I had this option.
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.