Arrakis is now fully merged back with Barrelfish! You can download the latest release from the Barrelfish website.
We keep the Arrakis mailing list alive for Arrakis-specific questions. We might defer answers to the Barrelfish mailing list if they are Barrelfish-specific.
Old release:
The old release of Arrakis is a fork of the Barrelfish research operating system, released under the MIT Open Source license. It can be found on our public GitHub repository. It was released on 2014-05-14, forked from Barrelfish release 2014-03-11. Its release announcement is on the Arrakis mailing list.