Some writers cannot write while music is playing, but I, fortunately, am not of that breed. I usually avoid music with lyrics while I'm writing, but that is certainly not always the case. So, without further ado, let me share with you some of my absolute favorites when it comes to writing music.
I love their music first and foremost because it is awesome—"epic" to say the least. It's powerful, atmospheric, and conveys a number of different tones that are all perfect for writing dark epic fantasy. But I also love their music because I don't really associate it with anything--while I'll occasionally recognize a track of theirs on a movie or video game trailer, most of their music is not on official soundtracks, but on their own albums. This means that, for all intents and purposes, their music is the soundtrack to my writing. That's how I think of it, anyway, and it's pretty cool to (pretend to) have your own soundtrack, I must say.
Their most popular album is Chronicles, and that is a great one, but my current favorites are actually Phenomena, Decimus, and Magnus: B-Sides. If you are a writer of fantasy of any kind, and you remotely like music at all, you NEED to check them out.
Soundtracks (particularly from Game of Thrones)
So, if you haven't listened to Sigur Rós, they are an absolute must. They are one of the true sources of beauty in this world. Buy an album of theirs, turn up the highest-quality stereo or headphones you own, turn all the lights off, find a comfy chair, and let their music take you.