Some references are explicit, such as Tom Hanks quoting lines from The Godfather (1972) while romancing Meg Ryan in You've Got Mail (1998).
Others may be more subtle: for example, Swingers (1996) references Jaws (1975) in the scene where Jon Favreau and Vince Vaughn are at a party and the familiar John Williams theme music from Spielberg's film is played in the background while they look around for girls.
And some are even more obscure: Terminator 2 (1991) references The Abyss (1989) because the name on the side of the liquid nitrogen-filled truck at the end of T2 is Benthic Petroleum, which is also the fictional oil company who owned the Deepcore underwater rig in the earlier film.