Apple has a special prize for whoever downloads the 25 billionth app from the App Store: a big, fat $10,000 iTunes gift card (which you can spend on apps, music, and books, or course).
There are two ways in which you can enter the contest. The first is simple - you download an app. The second is a way to enter without downloading anything. If you've got all the apps you need, you can enter with the alternate entry form.
The winner will be whoever downloads that 25 billionth app, or whoever submits the entry form as soon as the 24,999,999,999th app is downloaded. Apple explains it like this:
Sponsor is the sole judging organization whose decisions as to the operation of the Promotion and the selection of the potential winner are final and binding in all matters related to the Promotion. The prize will be awarded for the entry (either through an app download or through the non-purchase online entry) sent immediately following the download of the 24,999,999,999th app. Any download counter displayed by Sponsor is for illustrative purposes only. The potential winner will be determined by the order of the entries received. In the event that more than one entrant would be a winner based on the simultaneous timing of entries, one entrant will be randomly selected from those entrants as the winner. Each entrant’s chances of winning are dependent upon the number of eligible entries received.
Whether you are downloading apps or submitting forms, you can only do it 25 times per day, per iTunes account.
Apple says they will announce the winner in about 10 days after the 25 billionth download occurs. Somebody's gonna win, might as well be you, right?