The latest version uses a Mercurial repository and is licensed under the MIT Open Source license.
Barrelfish is not intended to be used as a commercial operating system, but rather as a platform to explore possible future operating system structures. It has been developed by the Systems Group at ETH Zurich and Microsoft Research since 2007 as a proof of concept. What makes the software special is the fact that can run on several hardware platforms, including x86 64-bit CPUs, ARM CPUs as well as Intel's single-chip cloud computer. The developers also highlight Barrelfish's capability to take advantage of heterogeneous processors that include, for example, GPUs.
The operating system is designed to run one instance on each processor, while each instance is able to communicate with each other via a proprietary message system to keep all instances in sync. We are not aware of any non-experimental uses of Barrelfish at this time - with the exception of the fact that the Barrelfish website apparently runs on a Barrelfish server.