[mirimir]

Yes. Each client runs a Tor .onion service, and there's end-to-end encryption. So arguably, users are mutually anonymous (except for their .onion addresses).

But it's only available for Windows, macOS and Linux. On Android, there's Briar, which is similar.[0] But Briar can also connect via Bluetooth and WiFi. That's useful when the Internet is unavailable. But it's bad because user anonymity could be blown.

0) https://briarproject.org/

[jstanley]

I was working on a self-hostable web interface for a while. There's a public instance at https://ricochet-web.org/ but unlike the official client it hasn't been professionally (or otherwise, for that matter) security-tested and is vulnerable in various ways the official client isn't. It also has quite a few known and unknown bugs.