Microsoft has a site that make creating Silverlight project mashups very easy. They just announced a new addition to Popfly that allows people to make game mashups. They have a bunch of templates that people can start with. The core of the games are Silverlight. You can even get to the XAML code and manually change things. Here is just a sample of what you can do with just the sample templates. I will be playing with this some more and converting a sample game that I have to use this, but for now, enjoy this sample.