The Xen Project, the community which develops the Xen hypervisor under the GNU Public License (GPLv) and which is now part of the Linux foundation yesterday announced the release of version 1.0 of the Mirage Operating System.
The Mirage OS represents an approach where only the necessary components of the OS are included and compiled along with the application into a "unikernel", resulting in more efficient and lean "appliances" which the same or better functionality but a much smaller foodprint and attack surface. Application code is developed in a high-level functional programming language OCaml on a desktop OS such as Linux or Mac OSX, and compiled into a fully-standalone, specialized unikernel that runs under the Xen hypervisor APIs. Since Xen powers most public cloud computing infrastructure such as Amazon EC2, this lets your servers run more cheaply, securely and finer control than with a full software stack.
The release notes for version 1.0 can be found here and the Mirage OS can be downloaded here