NIP-28. Amethyst had them built in and then there's also nostrchat.io and nostrchat.com and I believe Coracle used to have this built in, but no one used it so it was removed.

In Amethyst you have the "Chats" tab and the "Communities" tab. 0xChat does another type of public chat that Amethyst doesn't.

FOMO

Oh shit finally found it!! Nice!