Prosody - Prosody is a modern flexible communications server for Jabber/XMPP written in Lua.
Openfire - Openfire (formerly Wildfire) is a cross-platform instant messaging (IM) and groupchat server.
Matrix.org - Matrix is an open standard for decentralized persistent communication over IP.
Rocket.Chat - Rocket.Chat is a Web Chat Server, developed in JavaScript, using the Meteor fullstack framework.
MongooseIM platform - The massively-scalable XMPPJabber mobile messaging platform, for mobile and web, by Erlang Solutions
ejabberd - ejabberd is a robust, massively scalable and extensible XMPP server.