On Xbox One, Xbox Live Gold members can download Prison Architect: Xbox One Edition ($29.99 ERP) for free during the month of September. Livelock ($9.99 ERP) will be available as a free download from September 16th to October 15th.
On Xbox 360, starting September 1st, LEGO Star Wars III: The Clone Wars ($19.99 ERP) will be free for Xbox Live Gold members through September 15th. Then on September 16th, Xbox Live Gold Members can download Sega Vintage Collection: Monster World ($9.99 ERP) for free through September 30th.
As reported on Major Nelson