This is a question which always gives me a wildly different answer - depressing autobiographies, love flicks, certain perfumes, some games, etc. It's not something that necessarily makes you sad or cry, just something that hits a deep, emotional nerve within you. A lot of people who I talk to about this have their answers that stems from some fond childhood memory, or a time long past which it reminds them of.
In my case, this song called White Wine in the Sun by comedy artist Tim Minchin is what does it for me. This song comes from a man who spends most of his time on stage taking the piss out of religion and offending basically everyone, but then, in my case when I saw him live, finishes with this beautiful song. I was bawling my eyes out to the point where my girlfriend was asking if I was okay. I guess it's the deep love I have for my family mixed with the sincerity of such a song that comes out from a guy like this.
What is it for you? What's that one thing that, whenever you see or hear it, just stops you in your tracks?