Although I have not used it, I hear
eMusic.com is decent, although it is a monthly subscription service instead of a pay-per-download service. If you sign up for at least 3-months, it is $14.99/month. If you sign up for at least 12-months, it is $9.99/month. Not bad if you download at least 10 - 20 songs in a given month.
It also depends on the type of music you like though, they've got real/good artists....but they don't have a lot of really recent stuff. If you like older artists like Beatles, Clapton, etc.....you'll probably like their service.
I think they have like a 2-week or so Free Trial period that you can sign up for to see if you like their service or not. Might not hurt to give it a look.