Dexter to Supernatural

Nina Batista on Dexter comments on her husband Angel's two Doublemeat burgers from Buffy's Doublemeat Palace.

The X-Files? Cigarette Smoking Man and Buffy the Vampire Slayer's Spike both smoked Morley cigarettes.

Characters on Supernatural rented cars from the fictional Lariat Rent-A-Car which Mulder and Scully rented from on The X-Files