If you're okay with dealing with a foreign website format, then this one is for you. Whether its due to a glitch or just a real giveaway is unknown, but the Chinese Xbox service is currently giving away Assassin's Creed 3 for free. You just have to guide yourself through a few pages of Chinese to get it.
Here's what you do. You make a new email account at hotmail using their new @live.com email ending. Then you go to the Xbox Chinese webpage and attempt to download Assassin's Creed 3. It will then take you to a page to fill out your information. Name, address, phone number, yadda yadda yadda. Those aren't important.
When it gets to city and zipcode however, you need to put in Shanghai for your city and 100600 for your zipcode. And then hurray, you're done and can now download Assassin's Creed 3 to your Xbox. You can apparently play it in English after you download it as well, but I can't confirm that. I've included links to the store page and a translator below. Good luck. Credit goes to z7enZ at cheapassgamer for pointing this out.
EDIT: It has been patched, so if you missed out on it then I guess there's always next time.