Tegan & Sara...big time! Just discovered them a couple of weeks ago, and now I'm addicted.
Folksy, edgy, poppy, rock, a bit whacked, whatever. Sweet and funny twin sisters from Canada. Great to play the guitar along to. Takes repeated listening, but well worth the effort.
Their latest album is from 2004 (So Jealous) and is their most accessible. But they are not mainstream.