Apple's iTunes Store accounted for 69 percent of the digital music market during the first half of 2009, according to data from NPD. That figure lands the iTunes Store firmly in the top spot for music downloads, followed in a distant second place by Amazon with 8 percent of the market.

Apple is still holding strong in the overall U.S. digital and CD music market as well, accounting for 25 percent of music sold. "The growth of legal digital music downloads, and Apple's success in holding that market, has increased iTunes's overall strength in the retail music category," commented NPD's Russ Crupnick, Vice President of Entertainment Industry Analysis.

In comparison, Walmart holds the second place position for combined music sales at 14 percent, and Best Buy is in third place behind Walmart.

Apple's growth in the music market is growing at a healthy pace, too. In 2007, the iTunes Store accounted for 14 percent of the purchased music market, and in 2008, that number jumped up to 21 percent.

Despite the growing popularity of digital music sales, the CD is still the top dog in the music industry, accounting for some 65 percent of legal music sales. That lead, however, may not last much longer, according to Mr. Crupnik. "With digital music sales growing at 15 to 20 percent, and CDs falling by an equal proportion, digital music sales will nearly equal CD sales by the end of 2010," he said.