I quite like F# - it's similar to Haskell, but since it's a .NET language, you have an extensive STL and tons of 3rd party libraries. It works great with C#, so

I quite like F# - it's similar to Haskell, but since it's a .NET language, you have an extensive STL and tons of 3rd party libraries. It works great with C#, so you can switch between functional and OO programming depending on your domain, while keeping the same STL and technology stack. Also, you get the full power of Visual Studio (they recently changed it to be completely free), which can really help with productivity in my experience.