2K Games are running a publisher weekend promotion on Steam. Great news if you're looking to pick up great games like Bioshock Infinite, Borderlands 2 and Civilization V with discounts as high as 75% in some cases.
Furthermore, Xcom: Enemy Unknown is free to play this weekend (up until Sunday) and you only have to pay a quarter of the original price if you want to pick it up ahead of expansion Xcom: Enemy Within later this year. The discounts run until October 7.