I’m sure this information is in the IMDB somewhere, but I’m not sure how to find it. What song, not written for a movie, has been used in the most movies? I don’t mean Happy Birthday…a real song! 
Dang, I was going to say “Happy Birthday”.
I’ve heard “Born to be Wild”, “When a Man Loves a Woman”, and that “Oh Yeah” song used in several movies. No idea if they’re the most common though.
I was going to guess either Born to be Wild or Bad to the Bone.
Movies don’t often use Happy Birthday anyway because of the rights issues. Here’s a StraightDope column about it.
Anyway, hey you!, that link is absolutely terrific.
The link mentions that the song is still under copyright so a studio would have to pay royalties for using it in a movie. So I don’t see how that would discourage them from using that song. They’d have to pay royalties for any song written in the last seventy years or so (or since whenever the copyright is still valid). In fact, most movies use songs written since the 1960’s or so, or new songs written for the movie, so there are almost always still royalties involved. And even if the movie used some safely public domain musical composition, they’d have to pay some sort of performance royalties to whomever performed it.
Plus, while I’m not a movie producer, I can’t imagine that the music royalties amount to very much in the overall budget.
I don’t recall the details myself, but it’s been discussed on these boards before. Perhaps someone will pop in with the answer.
I’d guess that “Gaudeamus Igitur” must be pretty high on the list … any scene set in a University will be virtually certain to have it playing as part of the score.
Which reminds me that Pomp and Circumstance pops up a lot, but that’s because it really is used at so many graduations…
My WAG would be the Blue Danube Waltz. Which I hate, passionately.
How about Orff’s Carmina Burana, which seems to be used in every single epic battle scene?
Ahh, that would be Orff’s O Fortuna, the opening movement of Carmina Burana.