[matrix] is an open standard and communication protocol for real-time communication. It aims to make real-time communication work seamlessly between different service providers, in the way that standard Simple Mail Transfer Protocol email currently does for store-and-forward email service, by allowing users with accounts at one communications service provider to communicate with users of a different service provider via online chat, voice over IP, and videotelephony. It therefore serves a similar purpose to protocols like XMPP, but is not based on any existing communication protocol.
Beehub has a client running at https://element.beehub.io which also can be used to connect to other homeservers.
The next list are homeservers with public rooms available.
matrix.orgmedia.debian.socialarchlinux.orgubuntu.commozilla.orgmaunium.nethackint.orgcybre.spaceevents.ccc.de - CCC conference Hamburg, DEfosdem.org - Fosdem, Brussel, BEsec-t.chat- SEC-T, Stockholm, SEmatrixrooms.info (redirects you to other discovered rooms)