ConsolR is a really nice web-based C# execution environment, here is their description:
ConsolR enables you to execute C# code againt a running .NET 4.0 web application's app domain through a browser. Once installed, ConsolR will automatically configure itself during application start and is accessible through the "/consolr" path. This enables an interactive console session against for instance an application's production environment.
ConsolR is based on the Compilify project which provides the base REPL environment
Compilify uses the Microsoft's Roslyn engine which looks like a very powerful C# code analysis and manipulation (in fact from this Roslyn document I get the feel that I should rewrite O2's REPL Script environment using it)
Note: to install the VisualStudio I had to upgrade to the latest version of NuGet (which in VS 2011 SP1 can only be done after installing this HotFix)
Here it is in action:
Humm.... I wonder how much work it would be to convert this into an web version of O2 REPL environment :)