Ludii
A general game system whose Ludii game description language composes boards, pieces, turns, moves, and end conditions into immediately runnable rules, with built-in AI, self-play, and a concept database for analysis.
Why use it
Designers can implement abstract rule variants quickly, inspect legal play, playability, strategic depth, and recurring patterns, then decide which mechanics merit a physical or digital prototype.
Where it fits
Use prototypes and playtests to find core experience problems while they are still cheap to fix.
What to check
Its focus is discrete, turn-based, and combinatorial games rather than real-time action prototypes; source is published under CC BY-NC-ND 4.0 for noncommercial use, and the no-derivatives restriction does not meet the usual definition of open source.
