irssi - Irssi is a terminal based IRC client for UNIX systems.
SendBird - SendBird is messaging-as-a-service.
WeeChat - WeeChat is a fast, light and extensible chat client.
Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
KVIrc - KVIrc is a free portable IRC client based on the excellent Qt GUI toolkit.
Chat by Stream - Build and ship in app messaging in a few hours