Why buy only Diablo III when you can get Diablo III and StarCraft II: Wings of Liberty for the same price? No, that isn't a trick question.
The Blizzard Store currently has Diablo III and StarCraft II: Wings of Liberty available for $20 off. To make the deal even more fun, you get the difficult choice between either a physical copy to further clutter your room or a digital download. Either way you get a code that'll let you download the game from your PC, but the physical copy doesn't include free shipping.
Diablo III has received some beefy patches while StarCraft II is gearing up for its first expansion early next year, so there's never been a better time to get caught up on Blizzard's offerings.