Microsoft announced the XNA Game Studio Express a month ago. Now you can download the first beta version to start developing games for Windows XP and Windows Vista.

The link to deploy to the Xbox 360 is still missing in this release. Microsoft says that they do not want to have beta software on the Xbox platform. To use the XNA Game Studio Express you need to first have Microsoft Visual C# 2005 Express Edition installed, which is also free.
To make game development tools available to programmers at no charge is potentially huge for the Xbox 360. To activate and support the developer community is a strong point of Microsoft.