What is ScriptCS

Glenn Block and couple of more folks are working on a project called ScriptCS that allows developers to use C# as a scripting language. 

From http://scriptcs.net/ website:

What is ScriptCS?

Tool that allows you to write .Net and C# applications by simply using a text editor, NuGet, and the power of Roslyn!

What is Roslyn?

Traditionally, compilers are black boxes – source code goes in one end and object files or assemblies come out the other end. The Roslyn project changes that model by opening up the Visual Basic and C# compilers as APIs.  These APIs allow tools and end-users to share in the wealth of information the compilers have about code. The Roslyn CTP previews the next generation of language object models for code generation, analysis, and refactoring, and the upcoming support for scripting and interactive use of VB and C#.