Openfire - Openfire (formerly Wildfire) is a cross-platform instant messaging (IM) and groupchat server.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Bopup Communication Server - Secure instant messaging (IM) and private LAN chat solutions for organizing effective real-time communication system over office LANs, business and corporate large networks, or enterprise-size WANs and VPNs
WhatsApp - WhatsApp Messenger: More than 1 billion people in over 180 countries use WhatsApp to stay in touch with friends and family, anytime and anywhere.
ejabberd SaaS - Realtime XMPP and MQTT Software and Infrastructure as a Service.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.