As does C# with extension methods. It is especially useful when you have a generic class:

    class Foo<X> { ... }

    static class Foos {
      static void DoFooInt(this Foo<int> self) { .... }
    }

    new Foo<int>().DoFooInt();

I built a whole pimped library called "Bling" for C#/WPF to lift values into signal form around this technique.

http://bling.codeplex.com

Unfortunately, operators can't be externally extended in C#, so there is still a lot of wrapper creation, but extension methods can be used for that; e.g.

    Signal<int> a = ..., b = ...;
    var c = a.Bl() + b.Bl();

Bl is an extension method for signal ints that returns a wrapper around the argument that allows for access to the + method. Bl is overloaded for a variety of types to provide access to those wrappers.