Slack - A messaging app for teams who see through the Earth!
Glip - Glip is fully searchable, real-time group chat & video chat, task management, file sharing and more, in one easy-to-use app. Get started FREE today.
Mattermost - Mattermost is an open source alternative to Slack.
Spectrum Chat - A newly open sourced self hosted Slack like alternative for building persistent chat communities
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.