TapTalk.io - TapTalk.io is a complete in-app chat SDK and messaging API - Easy integration and customizeable.
Prosody - Prosody is a modern flexible communications server for Jabber/XMPP written in Lua.
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
Openfire - Openfire (formerly Wildfire) is a cross-platform instant messaging (IM) and groupchat server.
Chat by Stream - Build and ship in app messaging in a few hours
ejabberd SaaS - Realtime XMPP and MQTT Software and Infrastructure as a Service.