Missive - Team communication app now supporting FB, Twitter & SMS
Slack - A messaging app for teams who see through the Earth!
Slack for Mobile - Rebuilt from the ground up
9mCollab - Collaborate (efficiently) Conclude (fast) Contribute (in the knowledge base)
Tinder Swipe Surge - Get more matches by being at large events
Mattermost - Mattermost is an open source alternative to Slack.