Eivør Pálsdóttir. Pretty popular in Scandinavian countries, but fucking NOBODY has heard of her over here. She's absolutely amazing. Azam Ali is a close 2nd... she is a bit more popular, but for the most part still obscure.
Sufjan Stevens. He may not be entirely unknown but holy crap has he released some of the most cathartic music I've ever heard. I've listened to Djohariah 95 times and that song's 17 minutes long.
Mine would probably have to be Vibe Tribe, Morgan Page, etc.