Gajim - Full featured and easy to use Jabber client
Tox - Send messages, files, create groups and make Tox to Tox audio and video calls. Securely.
CoyIM - CoyIM is a safe and secure chat client.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Empathy - Apps/Empathy - GNOME Wiki!
uTox - µTox is the lightweight client with minimal dependencies; it not only looks pretty, it runs...