Slack - A messaging app for teams who see through the Earth!
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
Mattermost - Mattermost is an open source alternative to Slack.
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.
Flock - A faster way for your team to communicate