XNA Game Studio is the only thing I know about. It's free and apparently very robust, with a large support base. It's a Microsoft product, and essentially games created on it can be played on both Windows and the 360. Check 'er out here: http://creators.xna.com/Education/GettingStarted.aspx

Only thing is that I don't think it's too easy to share games you've created outside of the 'Creator's Club', unless your game is awesome enough to actually sell. Like The Dishwasher.