Apple's yearly developer conference should see iOS 8, OS X 10.10, and likely some hardware.
Apple Begins Offering Free Content in 'Apple Store' iOS App with 'Color Zen'
As in the leaked sample screenshot, the initial offering is Color Zen, a puzzle game typically priced at $0.99. The free content is accessed by visiting any of the pages for individual Apple retail stores in the app. Users do not need to be physically present in the store for the offer to appear.
Codes for Color Zen distributed through the app are valid through August 27.