Dropbox had acquired Zulip, a group chat app in April 2014. A year later, they have decided to make it open-source in Dropbox's hack week project. Dropbox has released the app under Apache license. For the uninitiated, Apache license is a free software license compatible with version 3 of GNU General Public License. It allows the user of the software the freedom to use the software for any purpose, to distribute it, to modify it, and to distribute modified versions of the software without any issue about royalties. Dropbox has released everything it owns regarding Zulip, including the server, Android and iOS mobile apps, desktop apps for Windows, Linux and Mac, and also the puppet configuration necessary for running the server in production.
The server and client code is already available in GitHub, and the Zulip site contains various hints on how to get started with building integrations and other enhancement to the app. According to Zulip's co-founder Abbott, IRC and XMPP have been around for a long time dominating the open source chat, but not development has been made on either of these two services in the last decade. He believes Zulip is better, with many useful features integrated and a well-engineered, maintainable codebase with scope for further development. Zulip does have competition as a standalone group chat app, with the app called Slick leading the pack. Threaded conversation is an important feature distinguishing Slack, along with Yammer and Convo, from Zulip.
Dropbox has had a renowned history of releasing a lot of software into open-source with the latest being the release of Hackpad collaborating text editing tool, following its acquisition. Dropbox has also released the zxcvbn password strength estimator, the Djinni cross-language bridging library and the Python JIT for Python as open-source. Recurse Centre, the free retreat for programmers, has also announced plans to work on the Zulip open source project. They were excited to try out some new experiments and maintain an active and sustainable open source community around the project. RC facilitator and alumna Allie Jones plans to hack on Zulip with interested Recursers, already having a local build up and running, tweaking and making experimental changes to the app to speed up parts of the interface.