Here's the one I was thinking about earlier...

It'll All Turn To Dust and We'll All Fall Down (http://verucasalt123.livejournal.com/181737.html) by verucasalt123 - The world had to end sometime. The Winchesters just didn’t know they’d still be hanging around to watch. Dean and Sam fuck their way across the desolate landscape of what’s left, clinging to each other in the face of having lost so much. But there’s hope where you least expect it.