Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Cisco Jabber - Cisco Jabber lets you access presence, instant messaging (IM), voice, video, voice messaging, desktop sharing, and conferencing.
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
Mattermost - Mattermost is an open source alternative to Slack.