Early on in the series, Dean's cell number is given, and for a while the studio had it set up so that anyone dialing the number would hear Jensen Ackles reading the message: "This is Dean Winchester. If this is an emergency, leave a message. If you are calling about 11-2-83, page me with your coordinates." The number is 1-866-907-3235.
The car Sam and Dean drive is a 1967 Chevrolet Impala, which has been nicknamed the Metallicar by fans of the show. The show uses five black Impalas. Dean has nicknamed the car "Baby".
While filming season two, Jared Padalecki (Sam Winchester), broke his wrist after performing one of the stunts on the show.
Many of the episodes are based on existing legends, myths, and ghost stories. For example the episode "Crossroads" is based in part on the legend of Robert Johnson - who in turn borrowed it from another Blues singer.
Many episodes are either named after movies ("Children Shouldn't Play with Dead Things", "The Usual Suspects", "I Know What You Did Last Summer") or classic rock songs ("In My Time of Dying", "Born Under a Bad Sign", "What is and What Should Never Be", "Sympathy For The Devil", "When The Levee Breaks").
The character of "Bobby Singer" was named after executive producer Robert Singer.
Originally Jensen Ackles auditioned for the role of Sam Winchester. When Jared Padalecki came in and read for Sam a slight recasting was done. Producers called Jensen and asked if he'd rather use his charm for Dean, saying he was the Han Solo character of the duo and what kid didn't grow up wanting to be Han Solo? Jensen accepted. It became known that he was previously interested in that role, because Dean was funny.
Riverview Hospital, a large mental health facility used as the main set in the season 1 episode Asylum, has also substituted for a prison, a regular hospital, and an apartment building on different episodes of the show. This is evidenced by seeing the same distinctive staircase in different episodes.
The Latin inscription on the barrel of the Colt says 'non timebo mala', which translates to 'I will fear no evil'.
Jeffrey Dean Morgan (John Winchester) is only 12 years older than his on-screen oldest son, Jensen Ackles (Dean Winchester).
Both Jensen Ackles and Jared Padalecki grew up in Texas - Jensen in Richardson, and Jared in San Antonio.
There is a particular clock that shows up in almost every episode.
An important aspect of the show's music is classic rock, most of which comes from creator Eric Kripke's own record collection, although one of his favorite bands - Led Zeppelin - has always been too expensive to license.
The characters Alastair and Crowley pay homage to notorious occultist Aleister Crowley.
The Impala had a license plate KAZ 2Y5 from Season 1 to Season 2. The KAZ is a reference to Kansas, the Winchesters' home state, and the 2Y5 is 2005, the year the show premiered. It was replaced in episode 2.20 with an Ohio tag CNK 80Q3. The creator of the show, Eric Kripke, is from Ohio.
(1) Internet Movie Database