The beta of XNA Game Studio Express has been released by Microsoft, and is available for free download at DirectX Developer Center (beta no longer available). XNA Game Studio Express beta currently only supports Windows game development, although support for retail Xbox 360 game development will available on final release, and will available upon purchase of a XNA Creators Club subscription. Also not include in the beta of XNA GSE are XNA Framework Content Pipeline feature, additional starter kits and tutorials.
To run XNA Game Studio Express, developers need to have Windows XP SP2 (Windows Vista only in future version release). Also required are Visual C# Express and the latest DirectX runtime updates, both of them available for download for free. To use XACT audio tool to modify or create audio files, full August 2006 DirectX SDK has to be downloaded and installed too.
Although XNA Game Studio Express will be available for free, it cannot be used to build a commercial Xbox 360 game. To develop and create a commercial Xbox 360 console game, XNA Game Studio Professional is needed where it will support the creation of commercial games on the Xbox 360 using an Xbox 360 developer kit. Titles created with XNA Game Studio Professional by developers with approved titles will be able to be submitted for certification. As mentioned above, to develop, debug and play games on the Xbox 360, XNA Creator’s Club subscription also needed to be purchased on the Xbox 360 through Xbox Live Marketplace at $99 a year or $49 for 4 months.
With the release of the XNA Game Studio Express beta, XNA partner GarageGames will begin enrollment for its Torque X beta program which can be accessed via msdn.microsoft.com/xna. Torque X includes Torque Game Builder, Torque Game Engine and a version of the Torque Shader Engine which have both been developed in conjunction with XNA Game Studio Express, allowing budding game developers to use drag and drop tools to easily create games.
More information about XNA Framework and XNA Game Studio Express can be found at XNA FAQ (no longer available).
Update: XNA Game Studio Express 1.0 Final