Here's all the the death cards that can be found in WaW, and where to find them.
Death cards are like cheats (similar to the Intel pieces on COD4) and can be activated through the settings on WaW and can only be used for private co-op matches (comes in really handy for private zombie mode matches).
There are 13 death cards in total, one card in each level, aside from the two vehicle missions.
Level: Semper Fi - Eight of Hearts
Effect: Steady hands pack a powerful punch. headshots cause enemies to explode.
Location: At the very beginning, after leaving the shack, you should see another shack to the right. It's in there.
Level: Little Resistance - Four of Clubs
Effect: Keep your finger on the trigger. Enemies take less bullet damage.
Location: This is at the end of the level. Before going into the last building, look to the right. It's in a corner of bushes.
Level: Hard Landing - King of Hearts
Effect: Go down fighting. Fire explosive pistol rounds while downed.
Location: Lower level of the big building. Before you enter this building there is a mini-trench outside. It's to the right when you go into the room with the stairs. About half-way through the mission.
Cold Dead Hands
Level: Vendetta - Five of Diamonds
Effect: Save your brass. Enemies take their weapons with them to the grave.
Location: After going through the sniper part at the first of the mission, Reznov leads you through a building. Follow the bar around to the end and there you go.
Sticks & Stones
Level: Their Land, Their Blood - Joker
Effect: Hope you've got a strong arm. Your weapon arsenal consists of a knife and a bunch of rocks.
Location: After destroying the tanks, you go through a barn. Search the stalls to your right.
Level: Burn 'em Out - Queen of Hearts
Effect: Steal life from your prey. Only recharge your health by getting kills.
Location: Just before the second mortar position, there is a bunker to the left. Inside is a Death Card.
Level: Relentless - Nine of Diamonds
Effect: No more easy grenade kills. Enemies take less explosive damage.
Location: This is at the end of the mission. You choose either to go down the left path or the right path. Eventually you'll run into a path that connects the two. Midway down the pathway is the Death Card.
Level: Ring of Steel - Jack of Spades
Effect: Hone your accuracy. Enemies die by headshot only.
Location: As soon as you enter the asylum, go the the left. It's in the far bottom left corner.
Level: Eviction - Ace of Spades
Effect: This is a different kind of war. Enemies are back from the dead with a vengeance.
Location: Well hidden. At one point you'll see a room to your right that has a huge hole in it. Just drop down and it's right there. It's in the first half of the mission, just after the kitchen area.
Level: Blowtorch & Corkscrew - Ten of Clubs
Effect: Make your weapon useful for a change. Shoot downed Co-Op teammates to revive them.
Location: This is to the left as you go toward the second bunker. You'll see a Japanese soldier hanging upside down near this one.
Level: Breaking Point - Three of Diamonds
Effect: You have an animal rage. Get three kills in five seconds to unleash the Berserker..
Location: After cleaning up all of the four mortar pits, head to the small shack in the south of this area. It's in there.
Level: Heart of the Reich - Six of Clubs
Effect: Can't handle the brutality of war? Try paintball guns.
Location: This is in the closed-off subway entrance across the street from the start of the mission.
Level: Downfall - Two of Spades
Effect: You've defeated the entire Nazi empire at the Reichstag. Now go back and do it again with the merciless nature of real war. Limit your HUD, turn on friendly fire, and bleed out in half the time.
Location: Before you get to the theater, a couple of your allies break through a door to your right. It's right in here.
EDIT: Forgot to mention that if you find all 13 death cards, you get a reward. On PS3 you get a silver trophy called 'Grave Robber'. On Xbox 360, you receive an achievement called 'Grave Robber' which is worth 15 points.