JESTER mentions Steve Goodman's "My Old Man", but I think Goodman's "Ballad of Penney Evans" is one of the saddest and most moving songs ever written, detailing the plight, (in a cappella), of a "young widow of the war that's being fought in Vietnam. And I have two infant daughters, I THANK GOD I HAVE NO SONS, now you say the war is over, but I think it's just begun". This song is a real tearjerker.Another of my favorite "sad songs" is Ian Tyson's BARNEY, about a rancher forced to shoot his old, decrepit horse. The song ranges thru nostalgia, reality, and depression. It's on an old Ian & Sylvia record, and evokes strong emotions: whenever I play it, people either love it or HATE it, ("how could you sing a song about something like that"?) Animal rights people tend to hate it, until they realize the song's not about killing animals, but rather BONDING WITH ANIMALS.
Both of these sad songs bring out exceptionally strong emotional responses, which a REAL sad song should.