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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.
http://todotxt.org/ fits most of the criteria I think.
Looked at etesync before. Etesync is kind cool! But tasks.org does not have iOS support.
NOTE:
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Hence, a higher number means a more popular project.