The best co-op game I played last year was Left4Dead and if you were among the few PC gamers that did not purchase this title, this is the perfect opportunity for you to try it, then buy it.  All day Friday, you are able to download Left4Dead on Steam and play the entire game including the recently released Survival mode.  After you try and fall in love with the game (as I know you will), you have all weekend to purchase it through Steam at 40% off.  

Left4Dead is a cooperative experience where four players are challenged through an array of campaigns attempting to escape the zombie apocalypse and make it safety.  The gameplay is a first-person shooter with a frantic pace and relies heavily on teamwork and communication.  You can also play a Versus mode where you have an opportunity to play as the special infected zombies, and Survival where you fight an endless zombie horde to see how long you can last.