iTunes Music Store

That’s technically true, but it’s not the whole story. iTunes is perfectly capable of converting AAC files to MP3s - if you set your CD ripping preferences to rip songs as MP3, you can right-click on any unencrypted AAC file, and iTunes will happily offer to convert it for you. But it refuses to do this for the tracks you’ve paid 99 cents for. Why? Because Apple wants you to buy an iPod.

Really? My understanding is that the encrypted AAC files sold by iTunes Music Store can only be played back with an iPod or with iTunes itself. A car stereo or DVD player that only supports unencrypted AAC files still won’t play the tracks you’ve paid 99 cents for.

That, and because there was no way the music labels would let Apple sell their stuff if Apple didn’t make at least a half-assed effort to prevent people from freely distributing the purchases willy-nilly.

I’m not sure if third-party AAC players will play back protected AAC files you purchased from the iTunes Music Store, but this is a good time to emphasize that there are two different versions of AAC involved here:

(a) Plain ol’ AAC, the not-proprietary-to-anyone standard that almost nobody knows about, and
(b) Protected AAC, the Apple-invented DRM that they sell at the iTMS.

And, of course, you can burn protected AAC files to regular audio CDs, then play those on any CD player, so you’re not entirely limited to using only iTunes or iPods.

eMusic sells real MP3s, and they seem to have plenty of popular artists: just on the preview screen I recognize Thievery Corporation, The Decemberists, Ray Charles, Bob Marley, The Kinks, Creedence Clearwater Revival, The Pixies, Green Day, Tom Waits, Elliott Smith, Duke Ellington, Charlie Parker, Ella Fitzgerald, Louis Armstrong, Patsy Cline, Ryan Adams, and Willie Nelson. Apple could surely sell at least some of their tracks in an unencumbered form if they really wanted to.

My experience with eMusic is that they have a very limited selection of most of the big names. I haven’t used it for about a year, so things may have changed.