This is a great game to reinforce automaticity with addition facts. Plus, it requires very little materials (just some index cards) so you can play it at home, too! Today we numbered our cards 1-9. You can really use any numbers depending on how challenging you want the game to be.
Two students sit (or stand) back to back. Player #3 sits off to the side with the deck of cards. Player #3 places one card on the other players' heads. He/she then adds the cards together and says the sum out loud. On the count of three, players 1 and 2 turn around and have to figure out what card is on their head. The first player to correctly state which card they are holding keeps both cards. This player then becomes the caller. Play continues until all cards are gone.
We did not play the game so that there was a winner. But if you wanted to, you could keep the same caller for the whole round and the player with the most cards at the end is the winner.