Apple devoted its home page to a big thank-you today, saying: “A billion thanks. 25 times over. The App Store has reached 25 billion downloads. Thanks for getting us there.”

The milestone was achieved by the users of more than 315 million iPhone, iPad and iPod touch devices worldwide.

In a press release, Apple said the 25 billionth app downloaded, Where’s My Water? Free, was downloaded by Chunli Fu of Qingdao, China; Fu will receive a $10,000 iTunes Gift Card as the winner of the App Store Countdown to 25 billion apps.

“When we launched the App Store less than four years ago, we never imagined that mobile apps would become the phenomenon they have, or that developers would create such an incredible selection of apps for iOS users,” said Eddy Cue, Apple’s senior vice president of Internet Software and Services, in a statement.

The 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. Apple says it has paid out more than $4 billion to developers.

The announcement was made just days before Apple is expected to reveal the iPad 3.