As part of their effort to hold on to people starting to realize that they might be getting more content per their dollar with Sony, Microsoft started the "Games With Gold" program, allowing a game to go free for two weeks (fifteen days-ish) that anybody with a Gold account can have for free.
The only problem is that while Sony's system tends to bring in games that are still relevant, Microsoft seems just willing to skim games that are several years old or Xbox Arcade games that they must feel not enough people cared about.
That isn't to say none of their games in this are good. I've just found myself more often than not downloading the games just so the system shows "paid" and I can get it in the future if the mood strikes me.
The newest game to become free to download is "Deadlight," a remnant from an old Summer of Arcade event featuring zombies that don't go under the name "zombie" because some worlds apparently don't have the word in their vocabulary and have to make up new ones.