We have seen lots of examples of original scores (music written for specific films). But of course, film directors often use pre-existing music for their movies. This often happens with pop songs. Here are a few famous examples: 

Christina Aguilera, Lil' Kim, Mya and Pink sang "Lady Marmalade" for the film Moulin Rouge in 2001. The original dates from 1974, performed then by the girl group Labelle. Let's watch the more recent version: 

                                                                        Christina Aguilera Lady Marmalade Licencia YouTube                                                                          

A film with interesting use of pre-existing music is Apocalypse Now, directed in 1979 by Francis Ford Coppola. It is about the terrible Vietnam war, in which many innocent native people died in the jungle as well as many soldiers on both sides. Coppola used the song "The End" by the American rock band The Doors to highlight the terrible damages caused by war. 

VirtualVisitor999 Apocalypse Now Licencia YouTube

In another film about war (this time about the brutal Kmer Rouge regime in Cambodia), The Killing Fields (1984) uses a classic anti-war song, "Imagine"  by John Lennon, for the final scene when the two main characters reunite. It plays a big part in giving an uplifting, emotional end to the movie. 

                                                                                                  David Archvadze The Killing Fields Licencia YouTube