This is what happens when you're trying to sneak up on a writing session. You know it's going to take a deep dive, a full plunge into the zone, and even though it's the thing you really want to do, you're dreading the energy it will take to get there. So you futz around reading your RSS feeds and such.
"The Great Gig in the Sky" is one of my favorite tunes, one of the few that can bring tears to my eyes. Because I'm not much of a fan-geek, I never knew the background of the tune. Turns out the vocal melody was completely improvised by Clare Torry, a singer they hired to come in and sing something over the track they had recorded. I had always assumed that the basic melody had been written, not made up on the spot while recording.
As a teen I was blown away by Dark Side of the Moon as an album, and "The Great Gig in the Sky" was the crown jewel for me. I played it for my Mom and she said, "How can you listen to all that screaming? That's not music." [Ed 10/7/2013: This just in from Mom. Her opinion remains unchanged 40 years later.]
You should listen to these with a good speaker system or headphones. Laptop or smartphone speakers just aren't going to do the trick.
The studio version from Dark Side of the Moon.
The making of. Warning: The audio on this is twice as loud as all the others, so you might want to turn in down.
An interview with Clare Torry about the experience of recording it.
Here's a killer live version with three vocalists, none of which are Clare Torry.