Korpiklaani is a band which sold the folk metal subgenre to me. I know they have a pretty big fanbase, but I'm pretty sure not a lot of people even among the fans have ever heard Shaman (Jonne Järvelä's previous band), which beats just about every other band at mixing folk and rock.
Also, Rootwater. They're a Polish heavy rock band, which experiments with nu-metal, folk, electro, just about anything they can get their hands on.
And, finally, the legend of Hungarian rock, Omega. The authors of the song which might have been the single best known rock anthem in Eastern Europe ("Girl with a Pearl's Hair", or "Gyongyhaju Lany") and a very good blues-rock band, virtually unknown west of the Oder.