F# is a variant of the ML family of programming languages, and began as implementation of the core of the ObjectiveCaml programming language by MicroSoft for MicrosoftDotNet. http://research.microsoft.com/fsharp/fsharp.aspx is the official website.

F# is an attempt by Microsoft to adapt functional languages to the .NET virtual machine. It is based on Caml, an older version of ObjectiveCaml, without the OO system (which is not a good fit with that of the .NET platform).  It is compiled to .NET bytecode, which is then compiled to native code by an implementation of the .NET CommonLanguageRuntime.  The performance does not on the whole match that of OCaml native code, but is often close.  F# supports a VisualStudio mode for development and debugging, as well as the .NET libraries for network, graphics, text, XML manipulation and database access.

F# is now an official .NET language, and will soon be releasing it's CTP.
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Related:
* Don Syme's Blog
** http://blogs.msdn.com/dsyme/archive/2006/02/25/539016.aspx
*** ''One of the things I like most about F# is how it allows typed FunctionalProgramming to be applied to data-rich domains such as MachineLearning, ComputerAidedDesign (CAD) and GeographicalInformationSystems (GIS). GIS is of particular interest to me as I learned a lot about this area back in my undergraduate days, and because of the ever-enduring appeal of maps and simulation. Like Kean Wamsley's F# and AutoCAD examples, it's just amazing how easy it is to get going with F# and ArcInfo.'' 
* ComparingCsharpAndFsharp
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CategoryProgrammingLanguage CategoryLanguageImplementation CategoryFunctionalProgramming CategoryDotNet CategoryEffSharp