Franz - Franz is your messaging app for WhatsApp, Facebook Messenger, Slack, Gmail, Telegram and many many more.
Gajim - Full featured and easy to use Jabber client
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
CoyIM - CoyIM is a safe and secure chat client.
Google Duo - Google's answer to Facetime, high-quality video calling on iOS and Android.
Empathy - Apps/Empathy - GNOME Wiki!