Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Mattermost - Mattermost is an open source alternative to Slack.
Gitter - Where developers come to talk.
Slack - A messaging app for teams who see through the Earth!
Flock - A faster way for your team to communicate
RingCentral Video - Live life unlimited. Free video meetings and team messaging in one app that works the way you do.