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Fairy-Stockfish
chess variant engine supporting Xiangqi, Shogi, Janggi, Makruk, S-Chess, Crazyhouse, Bughouse, and many more
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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.
But, if you have a variant which the free Fairy Stockfish or ChessV (Alt link) can not play, Zillions can probably implement it fairly easily. Just this year, I created a Chess variant using an unusual tiling of squares and triangles. The only engine which can play this variant is Zillions.
NOTE:
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Hence, a higher number means a more popular project.