This months first installment of the Games With Gold program which was announced at E3 in June - Defense Grid: The Awakening is now available for free until July 15th 2013 (Download at the end of this article).
Defense Grid: The Awakening is the definitive Tower Defense Strategy game that's easy to pick up, engrossing to play, and hard to set down. Strategically place your towers on the map to prevent alien enemies from stealing your power cores
The game usually costs 800 Microsoft Points and has multiple DLC available for it (unfortunately not free) - so completionists beware, you might have to fork out to get all of the Gamerscore from this! #BF0000">Some people are reporting that the game is still showing as 800 MSP on the website... unfortunately we don't have a workaround for this yet but we will update if we hear anything. #008000">Update - We can confirm this is now working properly - it can be downloaded for free right now!#LiveZone