Apple's dominant iTunes music store might have been first out of the gates with big-label, DRM-free music, but Amazon—now armed with more than twice as many copy protection-free songs as iTunes—is quickly closing the gap.

Yahoo! News reports that Amazon, which opened its DRM-free music store just six months ago, is now the No. 2 digital music seller, behind No. 1 iTunes.

It doesn't sound like iTunes is in immediate danger of losing its digital music sales crown—Apple claims it owns 80 percent of the digital music market—but Amazon's rapid progress must be pretty startling to Cupertino.

While Apple was the first major music retailer to start selling copy protection-free music from a major label (it announced a deal with EMI last April), it stumbled almost immediately by charging a 30-cent premium for DRM-free songs.

Apple finally dropped prices in the face of DRM-free competition from the likes of Wal-Mart and Amazon, but the damage was arguably done.

Meanwhile, Amazon got a leg up courtesy of Warner, Sony, and Universal, all of whom have been looking for a way to wrest some control back from iTunes and its virtual digital music monopoly.

All three signed DRM-free deals with Amazon—but not with Apple, meaning that Amazon's DRM-free catalog has swelled to about 4.5 million songs, compared to just 2 million for iTunes (according to Yahoo! News).

It's pretty ironic that the big music labels—which have, in the past, been morbidly afraid of piracy and digital music in general—have resorted to selling DRM-free music to get back at Apple.

But hey, it works for us music lovers who get to reap the benefits of all the competition.

Related:
Amazon takes on Apple with copy-protection-free music [Yahoo! News]