TapTalk.io - TapTalk.io is a complete in-app chat SDK and messaging API - Easy integration and customizeable.
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
Prosody - Prosody is a modern flexible communications server for Jabber/XMPP written in Lua.
QuickBlox - Cloud communication and data backend.
Matrix.org - Matrix is an open standard for decentralized persistent communication over IP.