If you've bought "Duels of the Planeswalkers 2013" by Wizards of the Coast and Stainless Games then you might have noticed a conspicuous option from the player status menu in the bottom right hand corner of the screen that says "promotional unlocks." Clicking or tapping it will yield another screen that has an "enter code" field where you get additional "Magic: The Gathering" cards to use in your decks for free.
According to a post on reddit.com by the user egibson, there are currently ten codes that Wizards of the Coast has distributed. Each code unlocks ten new cards for players to use in the deck builder, one for each of the decks. With ten codes there's 100 free cards in total, ten for each deck. This raises the number of customizable cards from 30 to 40. Not too shabby for free download content. In fact, it adds a significant amount of value to the game.
1 - FNMDGP
2 - KWPMZW
3 - MWTMJP
4 - WMKFGC
5 - NCTFJN
6 - GDZDJC
7 - HTRNPW
8 - FXGJDW
9 - PCNKGR
10 - GPCRSX
The codes work on all versions of the game. I personally used them on both the Xbox and iPad versions. If you encounter an error, try entering it with all caps.
Here's a list of all the cards unlocked by the codes, sorted by deck (provided by reddit.com user Toasty 117):
Born of Flame
Catch up on the "Magic 2013 Core Set"
"M13" event decks: "Repeat Performance," and "Sweet Revenge."
"M13" card lists: White, blue, black, red, green, gold, artifact, and land.
"M13" intro packs: "Mob Rule," "Sole Domination," "Depths of Power," "Path to Victory," and "Wild Rush."