Pirate metal
matttt has pointed out the existence of "pirate metal" band Alestorm. Judging on the samples on their MySpace page, they're fun but a bit generic-sounding. Based on brief listening, "Set Sail and Conquer" is probably the best track.

So, this means we have viking metal, pirate metal, troll metal, and even goblin metal. What other folk-tinged mildly comedic heavy metal subgenres remain to be invented?

• Troubadour metal
  
• Druid metal (admittedly, Spinal Tap were pioneers here)
  
• Robin Hood and his merry metal
  
• Highwayman metal (entirely based off "Stand and Deliver" by Adam Ant)
  
• Gypsy metal
  
• Cowboy metal (like Country and Western, but louder)
  
• Yodelling metal
  
• The Wurzels of Death Metal

Heck, several of these probably actually exist. (Particularly cowboy metal. Is that what Lynyrd Skynyrd are?) Maybe I would know about this stuff if I read Kerrang! or something.

I should note here that among my recent acquisitions is Tanzwut's "Ihr wolltet Spass" which is industrial metal with bagpipes, featuring Latin chanting taken from Carmina Burana and the works of medieval mystic Hildegard of Bingen. It's ace, and I will probably wax lyrical about it at more length later.