Hedgewars
open sourceWorms-style turn-based artillery with a web build. Free assets, free engine, playable in a tab.
Official siteThe viral lists of "play these games in your browser" are mostly unofficial builds of games that are still being sold, which is why half the links stop working. This page is the other thing: a list of classic and classic-feeling games that are genuinely free, either because the rights holder gave them away or because they are open-source projects with their own assets.
Two rules decide what qualifies. Either the people who made it released it free, or the project owns everything it ships. Nothing here needs a game file you have to supply from somewhere else, and nothing here is going to disappear because a rights holder objected.
Worms-style turn-based artillery with a web build. Free assets, free engine, playable in a tab.
Official siteTower defence crossed with factory automation, with a browser build you can play right now.
Official siteA polished open-source roguelike with a browser build. Traditional dungeon crawling, done properly.
Official siteid Software released the first episode of Doom as freely distributable shareware. That episode is genuinely free to play and share.
Official siteMojang keeps the original 2009 build online, free, playable in a browser, and it is theirs to give away.
Official siteEpic released the original Unreal and Unreal Tournament for free, permanently. The full arena shooter, not a demo.
Official siteOpen-source Transport Tycoon Deluxe with the assets included. Free forever, no original game needed.
Official siteA full free RTS in the Age of Empires mould, developed by an open project for over a decade.
Official siteFree 3D kart racer with its own characters and tracks. The closest free thing to Mario Kart.
Official siteThe developers open-sourced the whole game in 2004. Real campaign, real assets, free.
Official siteThe Settlers-style build-and-expand game, fully free and still actively developed.
Official siteRed Alert, Tiberian Dawn and Dune 2000 rebuilt. EA released those assets free, so this one is clean.
Official siteFast arena FPS descended from Nexuiz. Free engine, free assets, built for competitive play.
Official siteEscape Velocity-style trading and combat in space. Free, deep and quietly excellent.
Official siteOfficially free means the rights holder released it. Play it and share it, nothing to worry about.
Open source means the project owns its engine and its assets under a free licence.
Engine port means the engine is open source but the game is not, so you have to supply the data files from a copy you own. Perfectly legitimate, just fiddly, and a lot of viral links quietly skip this part.
Unofficial means a fan made it without permission. It works until it does not. Take-Two has taken down browser copies of GTA, and Nintendo has taken down every Mario port it has found. If a list sends you to one of those, expect the link to be dead within months.
Not possible means exactly that. Modern console games do not run in a browser. Any site offering one is a scam page or a download you do not want.