Starting August 1st, Crackdown will be free for XBL Gold members until August 15th. This game is normally sold for 800 MS Points ($10). And no, the Halo 3 beta is not included.
On August 16th both Dead Rising 2, normally $15, and Dead Rising 2: Case Zero, normally 400 MS Pts ($5), will be free until the end of the month.
You must be an Xbox Live Gold member to receive this free limited time downloads.
Halo 3 has been announced as a part of the Games for Gold program although no date has been announced.