Barrelfish: Microsoft's free multi-core OS
Credit:
http://www.barrelfish.org/
Microsoft Research and the Systems Groups at ETH Zurich have released the latest version of their research operating system, Barrelfish, under an MIT licence and placed it in a Mercurial repository for public access.
BarrelFish is an experimental operating system which has been designed to run on systems with heterogeneous CPUs, such as a computer with a mix of x86 and ARM, or a system on a chip with a variety of specialised cores. Currently the system runs on x86-64 bit and ARM multiprocessor hardware, Intel's 48-core research microprocessor (the Single-chip Cloud Computer) and BeeHivePDF, a many-core computer built using FPGAs.