Here are a few.
Spike in OMWF: 'The day you suss out what you do want, there'll probably be a parade. Seventy-six bloody trombones.'
"Seventy-Six Trombones" is the signature song from the 1957 musical play The Music Man (1957), written by Meredith Willson.
S6 Willow: "Fly, my pretty, fly. See what I did there?"
Reference to Wicked Witch of the West.
Warren in Normal Again "Got her tripping like a Ken Russell film festival,"
reference to director of Altered States. (Jonathan in the same episode references Jack Torrance of The Shining.)
In The Yoko Factor, Buffy makes a Godfather reference: "Family. What kind of family are you? Corleones."
In the same episode, Spike asks Adam if he's ever listened to the Beatles and Adam replies he likes Helter Skelter (from the White Album.) Spike responds that he's not surprised, which is likely a reference to murderer Charles Manson who believed the Beatles had hidden messages directed at him, and who named his killing spree Helter Skelter.
The opening scene from Storyteller is performed as if in a parody of the series Masterpiece Theatre.
"Oh Pacey, you blind idiot. Can't you see she doesn't love you?" from Out of My Mind.
Reference to Dawson's Creek
"Couldn't she have dressed up like Xena?" Willow in Halloween.
Reference to Xena the Warrior Princess.