The X Files was an influential TV show (1993-2002) following two FBI agents, Fox Mulder and Dana Scully, who investigate the unexplained. It has been mentioned in the Buffyverse on several occasions.
In The Pack, Buffy accuses Giles of trying to “Scully” her (Scully was sceptical about Mulder’s supernatural beliefs).
In Living Conditions, Buffy says about Kathy, “You’re right. Ooh! She’s even affecting my work, now. She’s the Titanic. She’s a crawling black cancer… She’s… other really bad things.” The “crawling black cancer” line is likely a reference to The X-Files. In the show, it’s an alien liquid organism which looks like oil. When someone is first infected by the black substance, it crawls up the body like bugs under the skin.
In Life Serial, the trio of nerds test Buffy using different methods, one of which being a time loop. Warren mentions a similar plot in an X Files episode “where the bank kept exploding”. The episode was called ‘Monday’ and was during season six of the show.
In Gone, Buffy says, “Xander and Anya are working on it. Mulder-ing out what happened.”
In Sanctuary, the following conversation takes place:

Kendrick: “Come on Kate. Everyone knows you’ve gone all Scully. Anytime one of these weird cases crosses anyone’s desk, you’re always there. We used to be friends - what’s going on with you?”
Kate: “Scully’s the sceptic”.
Kendrick: “Huh?”
Kate: “Mulder’s the believer, Scully’s the sceptic.”
Kendrick scratches his head: “…Scully’s the chick, right?”

The X Files returned the favour in it’s season nine episode ‘Daemonicus’ by referencing Buffy with the following dialogue:

Scully: “For the past eight years I was part of a unit known as the X-Files. Some of you may have heard of it.”
Cadet: “Ever slay a vampire?”
Scully: “Sorry to disappoint you, but this is a course in forensic pathology. Hard science. An X-File is a case that has been deemed unsolvable by the Bureau, because such a case cannot be solved it may beg other explanations… a vampire, perhaps.”