Mumble is an open-source, low-latency, high quality voice chat
The very last word in instant messaging
You can message and share files inside your intranet without a server.
Development toolkit for Web Services and XML data bindings for C & C++
Discord Messaging Panel that allows you to message inside of a bot
UFONet - Denial of Service Toolkit
Smoke: Android Crypto Chat Messenger
ChatSecure is a free and open source encrypted chat client for iOS
This Program Contains Many Useful Network Tools
Safer Instant Messaging
Self-hosted chat app for small teams
Secure chat and conferencing protocol
The original Xivio.com World Server Core
Underground IM is a free Java Instant Messaging solution.
GlowMuck is a multi-user text-based role-play platform, a MUD server.