Zulip is your open source alternative to Hipchat and Slack

Are you in need of a communications client for you and your team? Zulip is a free, open source option from Dropbox. The service doesn’t just let you chat with your co-workers; it also has integration with outside services. It even lets you share files and customize notifications, among many other features. You would, however, need someone on your team that can host their own server on an Ubuntu machine. If you have that up and running, Zulip then gives you access to clients for Android, iOS, Windows, and Mac.

Source: Lifehacker