25bn apps have been downloaded from the Apple App Store

"Apple® today announced that more than 25 billion apps have been downloaded from its revolutionary App Store™ by the users of the more than 315 million iPhone®, iPad® and iPod touch® devices worldwide. The 25 billionth app downloaded, Where's My Water? Free, was downloaded by Chunli Fu of Qingdao, China. As the winner of the App Store Countdown to 25 Billion Apps, Chunli Fu will receive a $10,000 iTunes® Gift Card.
[...]
The revolutionary App Store offers more than 550,000 apps to iPhone, iPad and iPod touch users in 123 countries around the world, with more than 170,000 native iPad apps available. App Store customers can choose from an incredible range of apps in 21 categories, including Newsstand, games, business, news, sports, health & fitness and travel. The App Store has paid out more than four billion dollars to developers."

Source:  Press release from Apple, 5th March 2012

Earlier - 10bn announced on 22nd January 2011

5bn announced on 7th June 2010

1bn announced on 24th April 2009