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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.
If you want to host something yourself and don't mind doing the setup work, you can run your own instance of PC2 (which was formerly used by the ACM-ICPC contest) or DOMjudge or DMOJ.
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.