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.
Conversations.im - XMPP client designed with ease of use and security in mind
Jitsi - Open-source video calling and chat platform.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
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.
Tox - Send messages, files, create groups and make Tox to Tox audio and video calls. Securely.