@switchingsocial@mastodon.at style post:
#Matrix (aka #Riot) is an easy, widely used free federated chat system.

You can find a moderated list of instances to sign up at https://matrixservers.net/servers/
Although I would recommend staying away from the main server, matrix.org, since it's quite overloaded.

The most popular, and finished client is Riot, which is available for web, desktop (electron), android and iOS.

Other notable clients are Nheko(C++/QT), Fractal(Rust/gtk) and Pattle (Android). There's also a lot of others in various stages of development: https://matrix.org/docs/projects/try-matrix-now

I'm also developing another webclient, which aims to be faster, and more like Telegram in terms of ui/ux. You can find more info at https://neo.pixie.town although I currently would not recommend using it as your main client.