Microsoft has confirmed that Magic the Gathering 2013 and Rainbow Six Vegas will be the two free games offered to Xbox Live Gold subscribers as part of Games with Gold for September.
Magic the Gathering Duels of the Planeswalkers 2013 is available to download for free from now until September 15. This will then be replaced by Rainbow Six Vegas on September 16 until September 31.
Both games will be free to download for Xbox Live Gold subscribers, and once you have them you can keep them forever, even if you cancel your subscription.