The world's most popular online music retailer, Apple's iTunes, reached a landmark milestone on February 25 when it sold its 10 billionth song.

The 10 billionth song, Johnny Cash's "Guess Things Happen That Way," was purchased and downloaded by US iTunes customer Louie Sulcer.

Apple's iTunes store has the biggest music catalog in the world, with over 12 million songs available for download. In 2008 iTunes overtook Wal-Mart to become the number one music retailer in the US. Best Buy, Amazon and Target lagged behind in the third, fourth and fifth positions.

In less than seven years since its initial launch, iTunes has grown to become the world's largest music retailer with some reports stating that it accounts for more than 70 percent of worldwide online music sales.