You already know what Finntroll is, but what is AMV?
An anime music video. . . consisting of clips from one or more anime television series or movies set to songs; the term usually refers to fan-made unofficial videos. Most are not official music videos released by the musicians, but rather amateur fan compositions which synchronize clips with a musical track.
Finntroll has more than a couple of AMV's using their songs. Trollhammaren, as you might guess, is far and away the most popular Finntroll song to get this treatment. Here is a cool Street Fighter vid with Trollhammaren as the music:
Here is Naruto,
Trollhammaren one and all.
And, while technically not anime in the strictest sense, here is Adult Swim's Dethklok, in a Trollhammaren video.
Here is a very nice Finntroll AMV using the song Grottans Barn, and video from Princess Mononoke.
Matthew Sweet's video for the song Girlfriend, might be the first AMV(from 1991), using video from the anime Space Adventurer Cobra.