Microsoft released F# under Open Source
F# is a functional programming language for the .NET Framework. It combines the succinct, expressive and compositional style of functional programming with the runtime, libraries, interoperability, and object model of .NET. F# is well suited to a spectrum of development tasks - from interactive, explorative scripting to component and large-scale software development. Visual Studio 2010 has built-in support of F#.
Microsoft earlier released F# PowerPack as Open Source, which includes collection of libraries and tools to use F# programming language. Recently they added the compiler code as 'code drop' as part of the PowerPack. The release is in Beta. The PowerPack include features such as a basic Matrix library and supporting math types, FsLex and FsYacc tools for lexing and parsing, support for using F# with LINQ-based libraries, and a tool for generating HTML documentation from F# libraries.
Code Drop model, where the compiler + libraries would be logically available for each release of F#. The current release is available as an MSI for use with .NET and Visual Studio, and as a ZIP for use with other CLI implementations and platforms, including .NET Compact Framework on Xbox 360, and Mono on Mac, Linux and other platforms. Don Syme, a Microsoft principal researcher who developed and maintains the code. Check out his blog for more details.