In a move very similar to Sony's last year, Microsoft announced today at its E3 media briefing that it will be adding monthly game downloads for Xbox Live Gold subscriptions. The promotion has already (apparently) begun with Fable 3, and more heavy-hitters are on the way next month.
On July 1, the "Games with Gold" promotion will add Halo 3 and Assassin's Creed 2. They'll only be available for a half-month window, and Microsoft has only committed to the promotion through the end of the year. It also notes that games vary by country and you might have certain games roped off if you're too young for them.