mosquitto - Eclipse Mosquitto is an open source (EPL/EDL licensed) message broker that implements the MQTT protocol versions 5.0, 3.1.1 and 3.1. Mosquitto is lightweight and is suitable for use on all devices
Chatter Script - Chatter is the first and most popular PHP chat script ever!
MQTT.fx - MQTT.fx is a MQTT Client written in Java based on Eclipse Paho.
Applozic - Chat API & In-App Messaging SDK for Mobile & Web Apps Innovate fast, save development cost and delight users with a scalable, secure and customizable Chat
HiveMQ - HiveMQ is the MQTT based messaging platform for fast, efficient and reliable data movement to and from connected IoT devices and enterprise systems
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.