Ripcord - A desktop chat client for Discord and Slack
Apache Vysper - Apache Vysper aims to be a modular, full featured XMPP (Jabber) server.
Slack - A messaging app for teams who see through the Earth!
Openfire - Openfire (formerly Wildfire) is a cross-platform instant messaging (IM) and groupchat server.
Ziggs - Smoothly Share Content Between Devices!
Matrix.org - Matrix is an open standard for decentralized persistent communication over IP.