Just noticed the http://www.tryfsharp.org/ which looks really good:
Here is their main REPL environment:
Which has a nice ‘lesson’ workflow:
Which guides the user to learn about C#:
Since I don't have Silverlight installed in Chrome, I tried it in Firefox and it crashed (part of the problem of running these PoCs in the client)
I like the way the lessons are been presented, and it would be great to apply some of those ideas to the online O2 Platform’s C# REPL: