I keep reading Supernatural/NCIS fic (about 10 so far) even though I've only seen one episode of NCIS (which was after about the fourth fic and just so I could find out what the characters looked and sounded like) and I've no real interest in watching more NCIS, but I really love Supernatural/NCIS fic and there are some really good ones out there. (will rec some if anyone's interested)

Also, I highly recommend this Supernatural/Harry Potter fic.
(Spoilers up to 4x17 tigger. Sorry. I'm not sure how far along you've got on season 4)