What I found interesting about Kodu Game Lab is that students can create games without the worry of having design or programming skills. This is good for creativity, collaboration and problem solving.
Teachers and students can take part in a Skype Lesson
labshttps://education.skype.com/projects/2428-microsoft-kodu-game-labs
Unfortunately it is not a web based program but a download.
http://www.microsoft.com/en-ca/download/details.aspx?id=10056
This is a Youtube video that demonstrates how to use program in 1 of 4 tutorials. You can go to youtube to view all 4 tutorials. http://www.youtube.com/watch?v=jg_jeN5AcPs