I don't know of any implementations for C#, but there's a whole new programming language based on the Actor model by Microsoft. It's called Axum:
Axum (previously codenamed Maestro) is a domain specific concurrent programming language, based on the Actor model, being developed by Microsoft. It is an object-oriented language based on the .NET Common Language Runtime using a C-like syntax which, being a domain-specific language, is intended for development of portions of a software application that is well-suited to concurrency. But it contains enough general-purpose constructs that one need not switch to a general-purpose programming language (like C#) for the sequential parts of the concurrent components.
