Tomorrow, Xbox Live Gold members will be able to download Might & Magic: Clash of Heroes, an excellent puzzle RPG from Capybara, developers of the upcoming Super T.I.M.E. Force for Xbox 360 and Below for Xbox One. It'll be available from October 1st-14th.
Closing the month out is Halo 3, a game Microsoft had promised would be free when the promotion was initially announced. If, for some reason, you're just catching up on the Halo universe, you can also pick up ODST as a standalone single-player game after you're done.