Folk metal is too way wide to specify something. Despite the ridiculous names that some bands calls their music, on folk metal you can find this types IMO:
-Folk acoustic/ambiental:
Ulver's Kveldsjanger, Empyrium (the 2 last albums), October falls, Tenhi, Uaral, etx.
-Black metal with folk infuence (where the black metal infuence is prevalent):
Ulver - Bergtatt/Nattens Madrigal
Falkenbach's ...En Their Medh Riki Fara..
Kampfar's Fra Underverdenen/Kvass/Heimgang
Graveland
Borknagar's selftitled
Taake
Enslaved
Windir, etx
-Folk metal with black (or another extreme metal style)influence (folk >black/whatever): I identify here 2 categories:
'Happy folk metal' like Finntroll, korpiklaani, Folkearth, eluveitie, ensiferum, turisas, etx.
Melancholic folk metal like Vintersorg (Hedniskhjärtad, Till Fjälls and Ödemarkens Son mainly - can be considered Viking metal too), Agalloch, Empyrium (they mixed doom, black and folk on their 2 first albums), Primordial, etx
-Viking metal (I separate it cause the sound and the music structures usually are different)
Storm
Isengard
Otyg
Falkenbach, since...Magni Blandinn Ok Megintiri...
Bathory's Hammerheart
Kampfar's Mellom Skogkledde Aaser
Einherjer, etx
-Progressive folk/black/whatever:
Lastest Enslaved/Borknagar/Vintersorg
Tyr
Solefald
Asmegin is weird to clasify but they're great too.
note: Amon Amarth is NOT folk, nor viking metal at all.