XNA is a game framework by Microsoft that allows deployment for Xbox Live Indie Games as well as Windows Phone 7.
How Do I Get Started With XNA?
Microsoft's App Hub site provides access to the XNA SDK, tools, samples and educational resources. Shawn Hargreaves' Blog provides details on the inner workings and advanced features of XNA. The XNAWiki is also an excellent resource for snippets and examples.
Supported Platforms for Game Development: