I just found this mono tool today – it’s included in the mono installer.
If you’ve added your mono path to the environment variables, simply type csharp in the command prompt, and you’ll get:
C:\Documents and Settings\morm>csharp
Mono C# Shell, type "help;" for help
Enter statements below.
And you’re good to go! I have a feeling that I’ll be using this feature extensively for both Mono and .NET development.