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.