SPN fic rec
Every Man's Got a Right by MollyC on AO3.
The fic is one of those rare ones that is Cas-centric and long, nearly 80,000 words.
Summary: 2014!Cas, after Dean's failed attempt to kill Lucifer in "The End", finds himself plopped down in the middle of the events of S2's "Croatian" with no idea how he got there or who put him there. He follows the boys through the balance of S2 hunts trying to change the past to prevent Sam dying at Cold Oak, and Dean making his deal. He's successful. Kinda.
I loved this fic because of how Molly C. got Dean and Sam to trust Cas without revealing he was an angel until the very end. We also get to see Cas reacting to a Dean that hadn't been to hell, and him helping the boys on S2's cases.
Two other SPN characters show up in non-canon S2 events as a surprise to help Cas out.