Folk Metal? Here comes my favourite band of all time: Agalloch. Pretty much the whole discography (which is not that big) but for more folky music I'd go for older ones, especially Pale Folklore and The Mantle. As for Viking Metal, I really enjoy many albums of Enslaved. But I really never even listened to all of them. But the most "vikingish" is Eld i guess. I like especially first song, 793, which is the year of The Battle of Lindisfarne, pretty cool stuff. I hope I helped ;)