Seriously, we should set it up. I bet someone else has, though. Anyone got a link? :)
Okay, here are some places I recently saw Supernatural guest stars:
An episode of Entourage from the last season, the one where Vince buys the dinosaur head. The actress who played Sarah in "Provenance" was in a similar role in the auction house.
Last week's episode of Leverage (two Sundays ago) had Todd Stashwick and the actress who played Casey in "Sin City."
I know there were others but I failed to write them down and my brain is starting to malfunction from hunger. I'm going to go make dinner--you tell me more in the comments!