Sigur Ros is great, they're one of my favorites for this application.
Some other good ones:
Band of Horses- The Funeral and I Go To The Barn Because I Like The
Explosions in the Sky
Any song really, though I find their newest one weaker. The problem with Explosions is that it does all sound really similar, but taken individually the stuff is great.
Slightly Stoopid- Wiseman, Collie Man, and Closer to the Sun
are my favorites, but it's all good for when you want a chill, less trippy mood.
Godspeed You! Black Emperor
Not extremely familiar with their discography, but I got some very good bootlegs off of archive.org. The files I have just have a date and a venue.