Conversations.im - XMPP client designed with ease of use and security in mind
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
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.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.