Slack is the hottest collaboration tool in the market today, both as an internal tool for teams working together and for external groups who share similar interest and need a place to collaborate and share ideas.
Slack also offers a free tier which is used by many public communities such as software developers and testers. This tier includes unlimited private and public channels, 10K messages, up to 10 apps (Github, Bitbucket, etc.), file sharing, custom notifications and more.