http://www.rainymood.com/

Start em both at the same time and you'll get some really nicely timed thunder through the song, adds a lot of atmosphere! I also found that this works great with Agalloch and Yob too.

Try it with The Death of Music (in a really good way)

An obvious one to try it with, but this works absolutely fantastically with Ki. Oooooh, this is delicious.

EDIT: The ending was perfect - a spot of thunder just as the guitar cut out.

I've been putting an mp3 rip of that on whenever I go to bed for the past few weeks, been working wonders for helping me sleep.

As for music to put it with, definately agree with Agalloch
http://www.youtube.com/watch?v=JidHHYDHBYk
Alcest works well with it aswell.