As part of its new Games with Gold promotion that was announced at E3 in June, Microsoft has stated that the next title that will be freely available to is Xbox Live Gold members will be Defense Grid: The Awakening. The game normally sells for 800 Microsoft points.
Defense Grid: The Awakening is an excellent tower defense game that received very positive reviews when it was first released back in 2009. The game will be available to Gold members from July 1st through July 15th after which time another game will takes it place.
At E3 Microsoft had announce that Xbox Live Gold subscribers would be receiving two free games each month that were "their's to keep." The promotion was obviously in response to Sony's PlayStation Plus service which also offers games to players.