I'm searching for a Sam/Dean fic I just recently saw. However since I wander randomly throughout the tags of this community I have no idea how recently it was actually posted.
 
It started out with Castiel going to a bar and meeting Lucifer. The two of them decide to spy on what the Winchesters are up to. At one point Lucifer was complaining about how much of a pain both Lilith and Azazel were for him. It takes place during season 4, since I remember off the hand remarks about Ruby.
 
Unfortunately I had just started it and don't remember anything else. I'm pretty sure it was a long story. I checked the spn_j2_bigbang post and didn't see it.

edit: found in comments