I have heard it said that drawing a distinction between Viking metal and folk metal is totally arbitrary. That the two are really just alternate names for the same sub-genre. Unfortunately, the labeling process is often complicated by this approach. I am not one who feels that everything needs to fit into a small, specific category. However, when trying to differentiate between two different things, having such labels available can be rather handy. Believe it or not, there are actually differences between these sub-genres, though they aren't always the type of distinct and exclusive differences you encounter between, say, death and power metal. It's more like the subtle break-over point between proto-black thrash bands and full black metal: sometimes the lines are blurry. Still, there are bands which clearly fall into specific categories within these styles. Bathory played Viking metal. Falkenbach play Viking metal. Korpiklaani, on the other hand, are clearly and distinctly a folk metal band. The whole "pagan metal" thing is really more of a re-labeling effort than a distinct style, but it has become a nice catch-all term for bands like Wolfchant who clearly fall into that general realm of music but don't readily fit into either of the previously existing categories. This is particularly true of bands whose styles and themes fit within the sphere of folk metal but who don't employ any folk instrumentation.
Between Viking and folk, though, there are often distinct differences. Viking metal is designed to sound "epic". Sound effects with clashing swords and crashing surf often appear. Songs are typically lengthy narratives. Deep-voiced male chants, battle-horns, large bells or gongs, slow tempos, and significant doses of reverb are all frequently employed to create mental images of golden halls and Viking battles. There are bands that take a faster, harsher, more aggressively stripped-down approach, but those are typically the bands that blend partially into the black metal scene. On the other end of the spectrum, significant use of violins and pipes, as well as more upbeat tempos, are sometime employed. Those are the bands that blur the lines into folk. Folk metal, in general, uses such instruments quite heavily, along with assorted other acoustic instrumentation native to their specific region, and while perky upbeat tempos are by no means essential, they are relatively common. Traditional folk melodies of the band's nation are often employed as well.
Really, the whole feel of Viking and folk metal are quite different from one another. For the starkest possible illustration of this difference, I would suggest listening to the songs "One Rode to Asa Bay" by Bathory, then "Wooden Pints" by Korpiklaani.
Well, with all that being said, it's rare to find somebody who loves Viking metal and hates folk metal, or vice-versa, especially considering that the preponderance of folk metal is from northern European countries and thus exhibits many similar tendencies to Viking metal. There are a number of interesting exceptions to that rule (particularly tribal metal) but I'll save that for another post. For now, I'll be lumping all my Viking/pagan/folk offerings together and presenting you with a handful of these bands which I really enjoy but which you may never have heard of before.
Gernotshagen - I love this band, or at least their present state. These Germans have only released 2 albums since their formation in 1999, and I'd be lying if I said their debut was brilliant. Their 2008 release "Märe aus Wäldernen Hallen", however, is fantastic and largely unknown.
Svafnir - This German project, primarily a one-man band, is responsible for a small scattering of recordings, including the excellent EP "Aufbruch". Surprisingly soft, and mostly instrumental, that record is a brilliant piece of music that is definitely worth hearing.
Galar - These Norwegians have released two full-length albums since their formation in 2004. Both are solid and worth checking out if you're looking to explore this area further. Since they lean quite heavily toward the black metal end of the spectrum, they make a nice transitional tool for people who aren't quite ready for the fiddles and pan pipes just yet.
Mistur - Another batch of Norwegians, Mistur finally released their full-length debut in 2009. Strom, guitarist from both Windir and Vried, is a member. The influence shows heavily. I would almost be inclined to call Mistur "Windir Jr", but that may be a little premature.
Grimm - Occasionally they'll play with synths to create a slightly unusual sound, but more often they're just a solid Dutch folk/pagan metal group.
Klabautamann - Germans who seem to have made a minor splash with their newest record, 2009's Merkur. Like Enslaved, it's kind of debatable whether they're far enough into the folksy side of things to really fit here rather than just being a progressive black metal band. Still worth checking out if you like either of those styles, though.
Grívf - A definite oddity, Grívf are a one-man Danish funeral doom/Viking metal band. It's a unique approach which frankly works extremely well in my opinion. Also rather oddly, Grívf has released only 3 full albums but did so in a span of less than two years in 2006-2007.
Nomans Land - These guys are a really catchy, fun Viking metal group from Russia. I'd personally recommend Hammerfrost as an extremely enjoyable record.
Nastrandir - One of those bands that falls into "pagan metal" for lack of a clearer title, Nastrandir are yet another pack of Germans. They're a good, solid, representative band within the generally blackened-but-definitely-not-pure-black realm of the Viking/pagan/folk metal world.
Kroda - I almost feel guilty including Kroda because of their Nazi-esque political stance, but musically they are just too good to exclude from this list. If you're not the type to let the personal activities of bands bother you, this Ukranian group is fantastic and warrants checking out immediately. "Fimbulvinter" is where I would go first, but as long as you stick to the studio albums, you really can't go wrong.
Assorted essentials: Bathory, Thyrfing, Vintersorg, Falkenbach, Finntroll, Månegarm, Ensiferum, Windir, Enslaved, Korpiklaani, Turisas, Skyclad, Cruachan, Suidakra, Eluveitie, Moonsorrow, and Ulver are all bands you should know if you're interested in this type of music. If there are any unfamiliar names here, I would strongly suggest you give them a listen. Some of these bands have morphed their styles at various points, though, so do a little poking around before you just grab something, especially if it's by Bathory or Ulver. I'm sure there are others I could list in this "essentials" category that are just escaping me at the moment, but if you're new to these sub-genres, there should be enough names here to give you a good start.