Free Microsoft XNA Game Studio Express
Microsoft has announced XNA Game Studio Express. A free set of tools based on the XNA platform to enable anyone to easily create games for both Windows and the Xbox 360 console. The free Express version lets you create commercial games for the Windows platform but you have to wait for the Professional edition next spring to create commercial titles for the Xbox 360.
It is not entirely free to develop for the Xbox 360 though. You must buy a XNA Creator's Club subscription ($99 for a year, $49 for 4 months) on Xbox Live Marketplace to develop, debug and/or play games developed for the Xbox 360.
The XNA FAQ has more information on which features are available in the upcoming beta.