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.
Spike - Spike solves businesses' communication chaos by bringing together email, team chat, collaborative docs, and meetings, into one feed.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Mailtrim - Email lives forever!. Reinvigorate your emailing experience with the best free email client, the Mailtrim