It appears Amazon.com has entered the mp3 sales biz, with over 2 million tracks available for purchase (covering ~180,000 artists represented by over 20,000 major and independent labels).
All songs sold are free of DRM (copy protection), using the .mp3 format, at a 256kbps bit rate (variable bit rate used in some cases). This means not only are you assured that these tracks will work on your current portable player and music software, but will continue to work on future players you purchase, no matter what kind you decide to own. Plus, at 256 kbps, you get high quality (CD quality) audio that will sound just as good playing through your headphones as through your stereo.
Prices are also excellent, with most individual tracks mostly priced $0.89 to $0.99. Albums are usually $5.99 to $9.99. As an extra feature, the "top 100 tracks" are $0.89 and the "top 100 albums" are $8.99.
You do not need to use any special software to purchase/download individual tracks, but in order to purchase an album, you need to download and install the "Amazon Music Downloader" (Windows and Mac OSX only). You can only download the music once, so make sure to make backups of anything you purchase.
It's unclear from what I could read as to whether these are available for international purchase, so your mileage may vary if you're outside of the US.