Even if you're not a fan of Supernatural this story is worth checking out. The plot, the world-building, the characters (and the sex) are just fantastic.
It's called Thursday's Child and it's written by Strangeness & Charm. It takes place after 5.04, The End, and is Cas-centric. It presumes that future!Dean did manage to kill Lucifer with the Colt. The problem with surviving the end of the world is that... you survived so now what do you do? What does a former angel do once the world is saved?
Warnings for language, character death, drug-use, non-graphic non-con, M/M and M+, religious themes (both pro and con) and a kinda-sorta cameo for Robert Downey, Jr. (yum!) Also, it's a fairly long fic so set aside some quiet time to enjoy. =]