If you're a gearhead and have a bunch of MP3 players from different companies, the Amazon MP3 Store is your best source for buying music downloads. It's the only store in which all tracks are unprotected MP3s, meaning they can be played on any player and in any software. (Microsoft's Zune Marketplace is getting close, but you need to download and install the Zune software to access that store, while Amazon is accessible from any browser.)
Yesterday, Amazon took a step toward making its store a place you might actually want to spend some time, rather than downloading your tunes and getting out. There are now 70,000 dedicated artist pages, each with a list of physical CDs for sale, most popular MP3 downloads, biographies from AMG, and listener-contributed information from SoundUnwound (an offshoot of Amazon's IMDb). If an artist has a store, it shows up at the top of search results--a nice touch given how haphazard Amazon's search results can be.
This isn't a new concept at all; iTunes has had dedicated artist pages for ages, and they're the centerpiece of MySpace Music. But you can't buy a physical album, DVD, or book from iTunes or MySpace.