Mirage is an exokernel for constructing secure, high-performance network applications across a variety of cloud computing, embedded and mobile platforms. Mirage OS was initially designed to for cloud use, which is why we call it a Cloud Operating System. Mirage OS applications are developed in a high-level functional programming language (OCaml) on a desktop OS such as Linux or Mac OSX, and is then compiled into a fully-standalone, specialised microkernel. These microkernels run directly on Xen Project hypervisor APIs. Since the Xen Project powers most public clouds such as Amazon EC2, Rackspace Cloud, and many others, Mirage lets your servers run more cheaply, securely and faster in any Xen Project based cloud or hosting service.

via Mirage OS.