Emusic is where I get most of the music downloads I use on my portable devices -- but I do have a Rhapsody subscription.
They offered a free trial, and I want to try most things free. I didn't expect to like it, though, as a long time hater of Real Player -- but I've gone ahead and paid my monthly dues. While the review hits them for their $0.99 price per track, that only applies to songs you want to transfer to your portable or burn to a CD. Your subscription allows you to listen to and/or download unlimited music on your home PC -- which works really well for me. I can listen to virtually anything I want.
I haven't touched iTunes -- does iTunes do this as well? I know Napster has a similar program.