Twinme - Twinme – Private Messenger is a free, secure, instant, and high-definition messaging app that protects your privacy along with giving you full control over each of your relationships and contents.

Empathy - Apps/Empathy - GNOME Wiki!

Textra - Textra is a seriously beautiful, feature rich SMS and MMS app.

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.

Material Messaging - Material Messaging is a beautiful communication app that lets you send and receive SMS and MMS messages using the intuitive interface while giving you complete control over every aspect of your messaging experience.

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.