Here's a Wikipedia list of pop songs that were inspired by and/or based on classical melodies. Most of us probably knew that Procol Harum's "A Whiter Shade of Pale" was based loosely on Bach's "Air on the G String," but did you know that Elvis's "I Can't Help Falling in Love with You" was origially a melody titled "Plaisir d'Amour" by classical composer Jean Paul Egide Martini (1741-1816)? I sure didn't.
How sad it is, though, that so many people copied Pachelbel's Canon in D. Many will beg to differ, but that is one piece of music I simply do not like.
Wikipedia hasn't listed "Somewhere Out There" (the theme from the animated movie An American Tail, sung by Linda Ronstadt and James Ingram) as being based on the second movement to Beethoven's "Pathetique" sonata. Maybe I'll write to the Wikipedia people and tell them to add it.