Microsoft has released XNA Game Studio Express and the XNA Framework out of beta as version 1.0, a games development tool designed to let game developers, designers and hobbyists to easily create and develop video games for Windows and Xbox 360 gaming platform using new and optimized XNA Framework, a set of managed code development cross-platform gaming libraries based on Visual C# Express 2005. XNA Game Studio Express 1.0 is available for free.
Although the game development tool for Windows and Xbox 360 gaming console is free, and XNA Game Studio Express can be used to design and develop Windows and Xbox 360 games, however, to develop, debug and/or play games on the Xbox 360, users must have an XNA Creators Club subscription purchased directly from the Xbox Live Marketplace, which costs $49 for a four month subscription or $99 for an annual subscription.
Download XNA Game Studio Express 1.0 which comes with XNA Framework, XNA Framework Content Pipeline, full set of documentation, how-tos, and starter kits that demonstrate how best to use the content pipeline and XNA Framework from
here (updated to Microsoft XNA Game Studio 4.0). There is also a FAQ that will answer all your questions about XNA.
Update: Microsoft XNA is no longer actively developed.