Slack is a cloud-based team collaboration tool.

Mattermost is an open source, private cloud Slack-alternative, Workplace messaging for web, PCs and phones. MIT-licensed.

Rocket.Chat free open source chat solution.

TeamSpeak is software for quality voice communication via the Internet.

HipChat is a web service for internal private online chat and instant messaging.

Qiscus provide everything you need to power up your app with chats.