Three down, one to go—Amazon just announced that DRM-free music downloads from Warner Music Group are now available to customers on Amazon's MP3 Store, further threatening Apple's near-stranglehold on the online music market.

Popular artists from Warner Music now available on Amazon MP3 include Red Hot Chili Peppers, Rob Thomas, Diddy, The Eagles, Green Day, Led Zeppelin, Madonna, and Metallica, among many others.

Warner Music Group now joins EMI and Universal Music in offering higher-quality tracks without copy protection, leaving Sony BMG Music Entertainment as the only major label refusing to drop DRM. This is big news; it means that cell phone and portable MP3 player owners can now buy online from a catalog of nearly 3 million tracks, confident that they can be moved among different devices without technical hurdles.

AMAZON MP3 ADDS MUSIC AUDIO DOWNLOADS FROM WARNER MUSIC GROUP [Amazon]