Apple iTunes
Downloadable program for Mac and Windows.

Rhapsody
Downloadable program for Windows, based more towards playing/streaming/listening than downloading.

eMusic
Subscription service where members can download up to 40 tracks per month of high-quality MP3 files.

Amazon
Yep. They now have an mp3 store!

Napster
Downloadable program for Windows. Yes it's now legal and paying.

Verizon V-CAST
V-Cast Music Store is Verizon's digital music service that can be accessed on a PC and on new Verizon phones. V-Cast includes music from the major record labels and independent artists.

Liquid Digital Media
Liquid exclusively provides music for Wal-Mart's digital download store.

PayPlay
PayPlay is a download store that provides widgets and focuses on independent music. It’s available for Windows users and requires no registration or software download.

AudioLunchbox
One of the first all-independent music download sites.

GroupieTunes
GroupieTunes is a service that sells independent music online and via mobile phones. Customers can purchase ringtones from their phones and have the charges included in their cell phone bill.

Ruckus
Ruckus is a subscription download service designed exclusively for colleges and universities. Participating schools pay a monthly subscription fee to provide their students with all-you-can-eat "tethered" downloads.