F# is a type safe, succinct, efficient, expressive functional programming language for .NET. You can find out all about F# today at F# Developer Center.
For F# 2.0 in Visual Studio 2010 Professional and above: Use any edition of Visual Studio 2010. For example, the trial edition of Visual Studio 2010 Professional.
F# 2.0 Free Tools for .NET 2.0, 4.0 and Visual Studio 2010 Shell: See F# MSDN Developer Center.
F# 2.0 Compiler on Mono/Linux/Mac/Windows: Install F# 2.0 for Mono/Mac/Linux/Windows (compiler binaries, ZIP, November 2010 release). Also see Installing F# on Linux and Mac, and consider the F# 2.0 addin for MonoDevelop.