#open-source
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Free chess, no ads, open source, with puzzles, analysis and a few million people who will beat you kindly.
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A Flash player rebuilt in Rust and WebAssembly, so the games and cartoons of 2000–2010 run again without the plugin. The reason half this list still plays.
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Console emulators for the browser, embeddable in a page. Bring your own legally obtained games; it brings the machine.