In Gears of War 3 for the Xbox 360, you can unlock the Aaron Griffin character as a skin to play as in the game.
To unlock Aaron Griffin, you just need to “like” the Gears of War Facebook page, and go to the Get Griffin link on the side menu to receive an unlock code to redeem on your console or Xbox.com. Here is the link directly to the page if you can’t find it.
Alternatively, you can get the Gold Big Money medal by acquiring $1,000,000 in Horde mode, if you’re not into Facebook.
Check out my other guide for more unlockable characters in Gears of War 3.