Payday 2 is still one of the most rambunctious cooperative games out there, a crazy crime spree where meticulous planning and execution usually goes to hell in a sea of smoke, SWAT teams and gunfire. It's an amazing heist simulator - but you don't have to take my word for it, because it's free to play on Steam over the weekend and only £7.66 to buy. To Overkill's credit, they've also taken the high road about the seemingly similar Battlefield: Hardline, which only emerged after Goldfarb and co. proved the concept worked.
If you're willing to buy from Amazon.com (and tell a couple of porkies about having an American address), you can undercut Steam's price further with a cool £5.98 price. Thanks to Webhead @ HUKD for both great spots!