Microsoft announced at their E3 press conference that they would be offering for their Gold Members something akin to Sony's Instant Game Collection for PlayStation Plus subscribers with their next Xbox Dashboard update. At the time, they said that Assassin's Creed II and Halo 3 would be the first games as part of this new program—called the Games with Gold program—but as a precursor to this update, they're releasing Fable III absolutely free for Gold Members.
Fable III will be free for all Gold subscribers until June 30th. On July 1st, the actual Games with Gold program will launch.
Unlike Sony's PS+ Instant Game Collection, Microsoft has said that once you download these games, they're yours "no matter what."