Quite unsurprisingly, Apple's online music retailer store iTunes ranked as the second biggest music retailer in the U.S., according to NPD Group's 2007 sales count. iTune's sales accounted to a six percent increase in digital music sales, aside from the video and movie content available in iTunes.

The sales increase has attributed to a decline in CD sales, however. According to Reuters, about a million audiophiles in the U.S. stopped buying music CDs in favor of digital downloads last year. This change of media preference pulled the music industry's performance down by 10 percent.

Apple's popular music retail store currently serves about 50 million customers, and digital music downloads make up about 10 percent of music sales in the United States. Approximately four billion songs were bought and downloaded from iTunes.