I've always liked the look of Amazon's burgeoning online MP3 store -- no DRM, no proprietary player, 280 kbps encoding rate -- there's almost no downside. You can buy individual tracks or whole albums at once, and their prices (for plain MP3's) are cheaper than iTunes.
So, when I saw that they required a special "downloader" to get tracks, I immediately thought, "Oh, great -- another DRM-free service that could've been perfect, were it not for the lack of a non-Windows software version." Because they totally wouldn't have bothered making a Mac or Linux version, right?
Not only do they have an Ubuntu version (that works perfectly, by the way), but they have specific Debian, Fedora, and OpenSUSE versions as well.
No .tar.gz files to mess with, no .bin's to get working.
Here it is on Ubuntu:
I'm guessing it integrates with Firefox on Ubuntu via just a simple MIME-type handling, since when you order a track on the site, the application automatically pops up.
Decisions like this one by Amazon to support Linux makes consumers such as myself much more likely to go to them when I'm looking to purchase MP3's, especially since without owning a Mac (or using a Windows PC), I'm unable to use iTunes. :P