are Amazon (http://www.amazon.com/gp/dmusic/help/amd.html) and Apple (http://www.apple.com/itunes/). If you have an iDevice (the iPod, iPad, etc) then go with Apple iTunes. If you have an non Apple MP3 Player, Amazon would probably be your best bet then.
Hope this helps!
I usually use Pandora and just listen to the free music there, but I want to get some permanent music. With Limewire totally shut down, and Rhapsody's whole "rent your song" thing, I'm kind of stuck.