While having fun with numbers (I need to read my RSS on a daily basis, I had hundreds pending) I tested some C# code snippets under SnippetCompiler, which is a tool that enables you to write code in C# or VB.NET and compile it without opening Visual Studio (and without having to create solutions or projects).
It's small, fast, and works with .NET 1.x and 2.0 frameworks. You can also add references and of course save your classes, so it's quite powerful. Very handy for those small code experiments!