Psi-IM - Psi-IM is a messaging program that is designed for the XMPP network.
Tox - Send messages, files, create groups and make Tox to Tox audio and video calls. Securely.
Gajim - Full featured and easy to use Jabber 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...