If you write 2D games for PC, Zune and XBOX using Microsoft XNA tools, then I have very good news for you: now you can play your games through the browser!
On Codeplex (hosting for opensource projects from Microsoft), a library has appeared that emulates XNA functions on the Silverlight platform, it is called
XN4SL .

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At the moment, XN4SL is in beta now at number 2.
A working example of a game ported to Silverlight can be found
here .
Reference:
Microsoft XNA is a set of computer game developer tools developed by Microsoft, which aims to facilitate game development.
Microsoft Silverlight is a browser plugin that allows you to run applications containing animation, vector graphics and audio-video clips, the main competitor of Adobe Flash.