F# is a strongly typed, functional-first programming language that lets you solve complex problems by writing simple code. Based on ML and built on the .Net framework, F# offers good interoperability, portability, and run-time speed as well as the "Five Cs" -- conciseness, convenience, correctness, concurrency, and completeness.
F# was initially available only on Windows as a Microsoft Research project, but it's now a first-class language on a number of platforms. You can use F# on Mac OS X and Linux with tool support in Xamarin Studio, MonoDevelop, Emacs, and others; on Windows with Visual Studio, Xamarin Studio, and Emacs; and on Android, iOS, and the Web using HTML5. In addition to general purpose programming, F# is applicable to GPU code, big data, games, and much more.