a good deal of overlap with the "romantic songs" thread:
Closer, by Nine Inch Nails: first kiss (it was onstage, in a play, where my character was supposedly seducing the guy I kissed; but I was terrified and had no idea what I was supposed to do)
Mahler's eighth symphony: I understood the Baudelaire poem we had to read in Humanites about getting drunk on Beauty to stop Time the first time I heard this.
Red is the Rose: the beginning of my first major relationship in college.
The Water is Wide: "I leaned my back against an oak, thinking it was a sturdy tree, but first it bent and then it broke as did my false love to me." Probably the end of this relationship. He's just sort of been ignoring me for the last month, which really takes some effort when we live across the hall from each other.

and more serious stuff: Almonds and Raisins: my sister and I played this for my grandmother in the nursing home; one of the songs she remembered/responded to the longest.
That's What Friends Are For: we sang this at my eighth grade graduation (K-8 school district) and I realized how fast time was going and how I would never see many of these people ever again.
Light One Candle: I sang this with a choir at the dedication of the Boston Holocaust memorial. Elie Weisel spoke. Before the official ceremony, a survivor talked to us in broken English about how he tried to look healthy enough to work. I remember the woman who lived down the block who had blue numbers on her arm. Julian of Norwich: Just repeating the mantra, All shall be well again, seems to help nearly anything.

and everything else I sing; these are the specifics I can think of now.

Chocolate Pi (it's Friday night; I must be coding!)