Grafton St buskers at their best are really really good, but there are also some very average buskers there every day too. New Orleans is a stand-out in the US where you can find world-class jazz bands playing on the streets.
Nashville has plenty in the evenings, and then you can find hot spots in some cities. I've seen regular buskers in Boston, Seattle, Sarasota, and Boulder - usually in pedestrianized touristy quarters.
