Growing up, I listened to Godspeed You! Black Emperor all the time. I always thought of them as a post-apocalyptic band -- pretty much perfectly used on the 28 Days Later... soundtrack when Jim is walking around London discovering how fucked everything has become. Great music for walking around late at night in the middle of a big city. It is a lot of dynamic variance and a lot of loose structure, oftentimes descending into beautiful, auditory chaos from a quiet few cello notes.
This video is perfect for the music. The track is called The Dead Flag Blues, and it features a great voice actor describing a ruined world over a series of Half-Life 2 visuals. This is pretty atypical of GY!BE, so listen to more if you don't dig into it right away.
Enjoy the next 5 minutes.