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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.
Also, while this uses leaked code, Valve has officially released the Source SDK 2013, which does include all the hl2 specific code, so if you copy over the assets from hl2, it can run just like normal.
NOTE:
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Hence, a higher number means a more popular project.