For this theme, someone almost always recs "In Reverse," so I guess it's my turn! It is a long and really wonderful story with Dean and Sam bumping into a group of Sam's friends while on a hunt and they all end up getting tangled up in the events. The hunt itself is creepy and the brother relationship is beautifully done. The OC's, Sam's friends, are interesting, dimensional characters. It's a really good read. If you haven't already read it, give it a try.

In Reverse (http://www.fanfiction.net/s/2787883/1/In_Reverse)