irssi - Irssi is a terminal based IRC client for UNIX systems.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Pidgin - Pidgin is an easy to use and free chat client used by millions. Connect to AIM, MSN, Yahoo, and more chat networks all at once.
Slack - A messaging app for teams who see through the Earth!
KVIrc - KVIrc is a free portable IRC client based on the excellent Qt GUI toolkit.
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.