For me it has to be 'Let my love open the door' by Pete Townsend. I had never heard of it before I watched Jersey Girl, but after hearing it, I completely fell in love with the song, and made me like the film more than I should have.
I have to say the same thing for the Juno soundtrack. The songs definitely enhanced the film and made me love the film instead of just liking it, especially because of Kimya Dawson and 'Anyone else but you' and 'Vampire'.