SPN Dean/Castiel, 'I, Castiel' by daymarket A really interesting robot AU, in which the angels and demons are robots. This is a bit too Philosophy 101 in places, and has an unresolved ending, but it's kind of awesome anyway.
SPN Dean/Castiel, 'Fooling Yourself' by notuslethe This is a surreal and creepy fic, with a dream-like writing style echoing the plot. People are dying due to their dreams, and it's just one more burden for a Dean on the edge of cracking.