Source.

Update: an Acapella cover by Peter Hollens:

I’ve listened to the snippet of this song in the Hobbit trailer maybe 40 times.

I’m part Scottish, that’s my excuse. Today, listening to this:

I was reminded of that great trailer, and listened to the song another 20 times. Of course, the trailer’s playing over it…still, a full minute of that minute point five trailer is all one song. That’s rare.

Then, just now, I thought to youtube search it. And hobbit song popped right up, me kicking myself, and lo, there it was. So here it is. Perhaps you’re as strange as I, and will enjoy half as much.

Far over the misty mountains cold
To dungeons deep and caverns old
The pines were roaring on the height,
The winds were moaning in the night,
The fire was red, it flaming spread;
The trees like torches blazed with light.

The bells were ringing in the dale
And men looked up with faces pale;
The dragon’s ire more fierce than fire
Laid low their towers and houses frail.

Note: just realized all the below cut off the drums or whatever at the end, and they’re the epic great part of the song, reminiscent of a Kurosawa film. Watch the full trailer for the full bit, though the trailer itself goes over the song a bit.

http://www.youtube.com/watch?v=v7rW6VWzHyw&feature=related Another full version, in case above copies get killed (three already have been blocked):

The song is called “Misty Mountains (Cold)” and was composed by Howard Shore, written by J. R. R. Tolkien, and is sung by Thorin and the dwarves in the film.

Seems like I’m far from alone, there’s a bunch of versions of this song, hacked off from the trailer somehow, each with 100,000 views:
http://www.youtube.com/watch?v=2v9YWPGiRLI&feature=related

This here version’s got it all:

 

Bonus:

http://www.youtube.com/watch?v=JDqtGgRFMEc&feature=related

Beautifully done, but not quite powerful or solemn enough: