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
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.
Miranda NG - Miranda NG is a successor of a popular multi-protocol instant messaging client for Windows - Miranda IM. Very light on system resources and extremely fast.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
IM+ - Chat everywhere! IM+ is the most widely used mobile IM app! Connect all-in-one to MSN, Yahoo!, Goo...