Samuel Barber's Adagio for Strings has been used just about everywhere!
(South Park, The Simpsons, Kevin & Perry Go Large, Red Dwarf, Platoon, The Elephant Man, etc, etc):
http://www.imdb.com/name/nm0053462/filmoyear
It was even used by Puff Daddy in "I'll Be Missing You" (although technically that was based upon Barber's own vocal version of the work, 'Agnus Dei')
However I suspect that you're thinking of this ;-)