Tinfoil Chat - Tinfoil Chat - Onion-routed, endpoint secure messaging system
Gajim - Full featured and easy to use Jabber client
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
CoyIM - CoyIM is a safe and secure chat client.
Tox - Send messages, files, create groups and make Tox to Tox audio and video calls. Securely.
Empathy - Apps/Empathy - GNOME Wiki!