If this topic has been posted before, I'm sorry I'm new here. Anyway, back to topic, the subject is pretty much explantory. I'll start with my favorite(s):

Rachmaninoff's piano concerto #2 in "Brief Encounter"
Beethoven's Symphony #9 in "Die Hard"
Orff's Carmina Burana in "Excalibur"
Duka's The sorceror's apperientince in "Fantasia"
Mascagni's Intermezzo from From Cavalleria Rusticana in "The Godfather III"
Barber's Adiago for strings in "Platoon" (Completely cliche)

Anyone care to add?