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.
Facebook Messenger - Facebook Messenger is a faster way to message.
Digsby - Tagged makes it easy to meet and socialize with new people through games, shared interests, friend suggestions, browsing profiles, and much more.
Jitsi - Open-source video calling and chat platform.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Empathy - Apps/Empathy - GNOME Wiki!