Sigfig said:
One of my latest preferred songs is Friday, by Rebecca Black. You should definitely listen to it a few times.
Its a Trap!
Also OP
Underground is a very broad, but if your referring to acts that would be considered Indie or tend to be grouped with Indie.
Radiohead, Sigur Ros, Arcade Fire and The Strokes
Underground as nearly unheard of groups that are really good.
Girl Talk*, Childs, Yuck!, !!! (band name), and The Antlers
Girl Talk's work can be downloaded free and legally.
http://illegal-art.net/allday/
Underground as is Punk
Joy Division, Rites of Spring, Cap'n Jazz and Pansy Division *Kind of a guilty pleasure of mine*
Underground as in Rap
Can't help you, not in a "I'm too holy for rap kind of way" but in a "I have yet to explore and find artists I like" kind of way.