The House of Hatrack has been running through some old Supernaturals and enjoying seeing all the one-off actors from other genre shows. Aside from the obvious, omnipresent Mark Shepard, so far (in the half dozen episodes we've watched) we've noticed:
Kaylee from Firefly
Larry from Eureka
Tara from Buffy
and Benny from season 8 pretending to be a vampire named Eli.
Anybody else you can think of?