It's not often you get something cool for free, most notably one of 2011's acclaimed shooters. That said, you shouldn't pass up a deal like this.
Best Buy has a promotion through its website and in stores where if you purchase one of its used games valued at $19.99 or more, you'll snag a free used copy of Call of Duty: Modern Warfare 3. Of course, quantities are limited, so you'll want to make sure your location has the game in stock. You can always check the website to see which stores carry it. The deal runs through Saturday, September 28th.