Empathy - Apps/Empathy - GNOME Wiki!
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Gajim - Full featured and easy to use Jabber client
Facebook Messenger - Facebook Messenger is a faster way to message.
IM+ - Chat everywhere! IM+ is the most widely used mobile IM app! Connect all-in-one to MSN, Yahoo!, Goo...
WhatsApp - WhatsApp Messenger: More than 1 billion people in over 180 countries use WhatsApp to stay in touch with friends and family, anytime and anywhere.