Korpiklaani haven't received enough attention in this thread.
They're somewhat silly, but their music makes for great party stuff.
The Hunting Song - pretty unforgettable
Wooden Pints - starring a
gorgeous fiddle solo
Beer Beer - yes, most of their songs are about drinking
Let's Drink - see above. Features some nice whistle playing.
Happy Little Boozer - somewhat more manic than their other songs
The thing about these guys (and most folk metal bands in general) that you have to keep in mind is that they're a heavy metal band first, and a folk group second. They play heavy metal and add the folk stuff on top. If you take out the folk instruments you'll still have some decent guitar-based music. They use a lot of folky chord progressions and modes, but ultimately they're a metal band and won't let you forget it.
Also hailing from Finland are
Moonsorrow who I don't think have been mentioned at all (please correct me if I'm wrong).
Essentially, Moonsorrow are the reason I became interested in folk music. They're like the aforementioned Korpiklaani mixed with John Williams, giving a really epic, symphonic take on folk metal. The growling vocals might be too much for some people, and it doesn't help that unless you're a fluent speaker of Finnish, it's unlikely you'll understand what on earth they're singing about. It's still great stuff if you're okay with that sort of thing. Some of their material is utterly breathtaking.
To ease you in, here's a beautiful folk tune, no metal, entitled
Kaiku that was the outro track on their album Verisäkeet.
Good stuff, huh?
Now onto their metal:
Sankarihauta is one of my favourite songs ever... majestic and epic in every way.
Sankaritarina is less folky and more epic, but you have to respect any metal band that can actually make a jaw harp sound cool. This sort of stuff has had a huge influence on my band. I think the lyrics have something to do about warriors going to heaven, or somesuch thing. In any case, the music says it all really.
And here's a pretty awesome cover they did of
For Whom The Bell Tolls by Metallica, who as far as I know don't have any folk influences (although it has been rumoured that Lars Ulrich is, in fact, a leprechaun.)
And another band worth looking at are
Alestorm. They're Scottish, and sing about pirates. To be fair, I think they're more about the gimmick of pirates and stuff, but they do have some lovely sounding melodies and progressions, plus you can't beat them for sheer fun.
Song titles include
Keelhauled which actually includes the lyric 'Keelhaul that filthy landlubber, send him down to the depths below/ Make that basmati walk the plank with a bottle of rum and a Yo-Ho-Ho' as a chorus.
Wenches & Mead describes their agenda accurately enough.
And finally, here's them interpreting
Flower of Scotland. I'll let you draw your own conclusions.
So there was NeonVomit's guide to Folk Metal. Not bad for a second post!