Ravenfall is an idle role playing game that a Twitch streamer runs and their whole chat plays. The streamer installs the client; everyone else joins by typing !join. There is nothing for a viewer to download and nothing to sign up for.
Characters carry on without you. You pick a skill to train and it keeps going while you watch something else, and the progress is yours: the same character plays on any channel running the game, so what you build on one stream comes with you to the next.
Around that sit the things that make a chat play together rather than side by side. Raids and dungeons the whole stream joins at once. Clans that share a level everyone contributes to. A town per streamer whose plots boost a skill for everybody playing there. A marketplace where players trade with each other rather than with the game.
It is free, and it stays free. The Patreon pays for the servers and gives patrons an experience multiplier and a few conveniences; the tiers page lists exactly what, and nothing there is unavailable to anyone else with time instead.