For those with Xbox Live Gold Memberships, every month Microsoft tags a few games and offers them as free downloads. The Games with Gold program offers two free games to Xbox Live Gold Members each month.
One of the two titles becomes available on the first of the month and the second title lands on the 16th. For November, your Games with Gold titles are A World of Keflings (available on the 1st) and Iron Brigade (available on the 16th).
A World of Keflings normally runs $9.99 and is a fantasy game where you take your Xbox Live Avatar on a journey through mystic lands while you help tiny Keflings (think Gulliver's Travels) build and customize their kingdoms. The game can be played offline with up to two players in co-op style or online with up two four players.
Iron Brigade also normally runs $9.99 and is a tower defense shooter that calls on you to fight your way through European, African and Pacific theaters. You take advantage of a combination of mobile firepower and stationary defensive units to defeat invading armies.
You can find all the details on the Games with Gold program here at Xbox.com or from the Marketplace on your Xbox console.