Today through Aug 31st, Dead Rising 2 (normally $29.99) and Dead Rising 2: Case Zero (normally 400 Points, aka $5) are free for XBL Gold members. Crackdown was previously available from Aug 1, 2013 – August 15, 2013.
The “Games with Gold: program gives XBL Gold members 2 free games each month.
Halo 3 has been confirmed as a free download but has not yet been given an availability date.