Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
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.
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
Riot.im - Open source interoperable collaboration app.
Jitsi - Open-source video calling and chat platform.
Slack - A messaging app for teams who see through the Earth!