Songs that were not original to a movie's soundtrack - but used so perfectly that I absolutely can not hear them without thinking of a particular movie scene.
  1. Boogie Nights, "Machine Gun" (Commodores)
    This movie is packed with unforgettable music scenes - "Sister Christian," "You Got the Touch," "Best of My Love," etc. But I don't think anything could top the cast all dancing to "Machine Gun" in the montage. I almost wished the movie had ended right there - unsurpassable.
  2. Fast Times at Ridgemont High, "Moving in Stereo" (The Cars)
  3. Beasts of the Southern Wild, "Until the Real Thing Comes Along" (Fats Waller)
  4. Jackie Brown, "Natural High" (Bloodstone)
    When Max Cherry first lays eyes on Jackie Brown and this song plays - it's a perfect example of song usage being such a slam dunk that no further explanation is needed: They are meant to be. (Also incredible use of "Across 110th Street" and "Strawberry Letter 23".)
  5. Bernie, "76 Trombones" (The Music Man soundtrack)
    Kind of cheating here 'cause it's Jack Black's version, but hey.
  6. Planes Trains and Automobiles, "Mess Around" (Ray Charles)
    And the Flintstones theme, of course.
  7. Dumb and Dumber, "The Rain, the Park and Other Things" (The Cowsills)
  8. The Big Lebowski, "The Man in Me" (Bob Dylan)
  9. Crazy Love, "Love Hurts" (Everly Brothers)
    This is a pretty obscure film adaptation of Bukowski stories with one unforgettable moment: We pan up the body of a teenager in the 50s, lying in bed and listening to "Love Hurts." When the camera reaches his face, he's completely covered in a nightmarish amount of acne.
  10. The Hangover, "Fever" (The Cramps)
    I swear, this spectacular song was a key factor in the movie's success.
  11. Trainspotting, "Lust for Life" (Iggy Pop)
    Only ranked above "Perfect Day" and "Born Slippy" because this use of "Lust" basically makes you love unlovable characters for the entire rest of the movie.
  12. Halloween, "Mr. Sandman" (The Chordettes)
    Not really a "scene" cause it's over the end credits. But I could never hear it without thinking of Michael Myers disappearing at the end of the movie.
  13. Casino, "Jump Into the Fire" (Nilsson)
    This would have to tie with its use of "Can't You Hear Me Knocking."
  14. Royal Tenenbaums, "Needle in the Hay" (Elliott Smith)
  15. Return of the Living Dead, "Partytime" (45 Grave)
  16. Goodfellas, "Layla" Eric Clapton
    Suggested by @CarlosHerrera
  17. Three Amigos "Little Buttercup"
    Suggested by @johannasamuels
  18. Magnolia "Wise Up" Aimee Mann
    Although people are divided on this.
    Suggested by @johannasamuels
  19. Dazed and Confused, "School's Out" (Alice Cooper)
    Suggested by @carolineschwartz
  20. Almost Famous, "The Wind" Cat Stevens
    Several choices from this movie, of course. America by Simon and Garfunkel and Tiny Dancer - both great but less subtle.
    Suggested by @hollisbrown