The CLI itself actually supports unadorned functions, but C# doesn't provide a way of writing them, or using directly them without resorting to reflection. I'm pretty sure C++/CLI lets you use and write standalone functions in managed code, and come to think of it I imagine F# would too. Also it's not an exact equivalent, but C# extension methods can often be used instead of static methods.

You can mark a class as static, then it will only allow static functions or members:

static class Math { static int Pow(int a,int b) {...} }