Yo La Tengo are fantastic but hugely unknown all things considered.
Joanna Newsom should be far more popular than she is.
Paloma Faith - most Brits will have heard her music on an anti-drinking advert but she's VERY unknown (but she hasn't released an album yet so fair enough...)
Jeffrey Lewis is a fantastic, amazing anti-folk artist.
Dan Le Sac vs Scroobius Pip are a great hip-hop outfit, as is M.I.A. (though she's somewhat more popular now that Paper Planes - one of her worse songs - is a bit of a hit)
Will Oldham, Willy Mason, Ani DiFranco, Boards Of Canada, Howling Bells etc.