Slack - A messaging app for teams who see through the Earth!
Missive - Team communication app now supporting FB, Twitter & SMS
Mattermost - Mattermost is an open source alternative to Slack.
Pingly for iOS - A beautiful, modern email provider for today's internet
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Front - multi-channel inbox service for collaborative teams