A little something I did for a friend for fun which was basically like:
"Ohey, you are writing a really nice novel there. lemme give you a Cover for that so the PDF isn't plain letters anymore"
"We got a Story and a Cover. What stops us to print it anyway?"
"I slipped and - oh what's this - a LAYOUT in InDesign."
"My Finger aciddently hit a upload-button at a printing shop. here is your copy."
Well, this escalated quickly.

This was the most uncomplicated collaboration I EVER HAD. It felt so natural.
See the result here:
http://gaiasangel.tumblr.com/post/90334410650/165-pages-of-awesome-a-little-private-project