Day Z is a zombie themed game mod for the game Arma II: Combined Operations. The game is still in the Alpha stage, but the developers have announced that the game is nearing 1 million unique players. The game is free and shows that developers of games, like those behind Arma II, can benefit from including mod tools in their games. The mod has helped to sell thousands of additional copies of the game and even put Arma II at the top of the Steam Summer Sale leader board, despite being on sale for only 20% off for much of the sale. The developers behind Day Z are rumored to be working on a standalone game that could bring them financial success, much like famous game mods like Counter-Strike in the past.