The App Store's one billion app countdown
When you download an app from the App Store (either in iTunes or from an iPhone or iPod touch) you're automatically entered for a chance to win. There's even a way to enter without purchase. Official rules are posted on Apple.com
As part of the promotion Apple has resurrected its list of all-time top 20 apps both paid and free. Which is worth a look.
Update: Apple's app counter has been called fake but it turns out that it's simply a very smart background change of this image that simulates the counting pace according to the latest stats from the App store.