Microsoft has already started rolling out its Games with Gold promotion for Xbox Live Gold members. The monthly offerings will be released on the 1st and 16th of each month going forward, until the end of the year.
The promotion page (via Joystiq) detailed the dates. Fable 3 is currently on tap, and you can grab it through June 30. Halo 3 and Assassin's Creed 2 have already been announced as July's games, though Microsoft hasn't detailed which will come first.
Microsoft has only committed to this promotion through the end of the year, though we'd expect it to keep going if fans react positively to it. It also distinguishes itself from Sony's similar PlayStation Plus promotion by promising that once you have the games, you can keep them forever "no matter what." That implies that the games are still yours even if your Gold subscription lapses, unlike PS Plus which requires a constant subscription to continue accessing freely downloaded games.