irssi - Irssi is a terminal based IRC client for UNIX systems.
Jitsi - Open-source video calling and chat platform.
KVIrc - KVIrc is a free portable IRC client based on the excellent Qt GUI toolkit.
Trillian - Trillian is a decentralized and federated instant messaging platform that lets your whole company send private and group messages, keep tabs on what co-workers are doing, share files, and much more.
mIRC - mIRC: Internet Relay Chat client
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.