Ruffle
What it is
Ruffle is an open-source emulator for Adobe Flash Player. It re-implements enough of the Flash runtime — the SWF file format, ActionScript, and Flash's rendering and sound behaviour — to play back existing Flash content in a modern browser, without installing the original Flash Player plugin that Adobe Flash itself relied on and that is no longer supported or distributed.
Ruffle is written in Rust and compiles to WebAssembly for use inside a normal web page, and also ships as a standalone desktop application and browser extension for playing SWF files directly. Because it runs inside the browser's own sandbox rather than as a native plugin, it avoids the long history of plugin-level security problems that helped end Flash Player's life in the first place.
Why an emulator instead of the original plugin
By the time Adobe ended support for Flash Player (see The Flash shutdown of 2020), the original plugin was no longer being patched, was blocked outright by Adobe's own kill switch, and was increasingly incompatible with how modern browsers handle plugins at all — most had removed old-style plugin support years earlier. That left a genuine problem: an enormous amount of creative and historical work, from art and music to entire games, existed only as SWF files built to run inside that one now-defunct plugin.
Rather than trying to keep the original, unmaintained, insecure Flash Player alive, Ruffle re-implements the format from scratch as a compatibility layer. This is roughly the same idea as an emulator for an old game console: you don't need the original, aging hardware (or, here, the original plugin) to run the software, only something that faithfully reproduces how it behaved.
Coverage
Ruffle does not, and does not claim to, reproduce one hundred percent of Flash's behaviour. ActionScript is a large and, in places, loosely specified language, Flash content spans well over a decade of authoring tools and quirks, and some content leans on features that are difficult to emulate exactly, such as particular video codecs or edge cases in text rendering. In practice, though, the huge majority of ordinary Flash animations and games — the kind sites like this one catalogue — run correctly or very close to it, and the project actively improves compatibility over time as real-world content is tested against it.
Where it's used
Ruffle is used by sites and archives that host historical Flash content and want it to keep working for visitors without asking anyone to install anything. Newgrounds, one of the largest surviving libraries of Flash animation and games, adopted Ruffle to keep its back catalogue playable. Preservation projects such as Flashpoint bundle Ruffle, alongside other emulators for other formats, as the way archived SWF files are actually played back rather than just stored. This site's own game pages use Ruffle the same way: once a catalogued game's file has been archived, its page swaps in a Ruffle player instead of the "not yet playable" notice every uncollected game currently shows.