If you're a game developer who already has your XNA games running on PC or the Xbox 360, you should easily be able to port these games for playing in the browser using the SilverSprite library on CodePlex.
SilverlSprint help you run your XNA game in Silverlight 2, though there is a limitation: the games can have SpriteBatch-based graphics, meaning no 3D games. Since XNA Game Studio is C#-based and Silverlight can do C#, it is possible to easily port an XNA game to the browser using the SilverSprite library. SilverSprite simulates the XNA Framework library, meaning there are no code changes to the XNA games required.
If you are a small game studio, hobbyist, or student looking to get started in game development or find resources to help a game concept already in development, see XNA Creators Club Online.