Some of these may not be SAD, but more on the emotional side
"If You're Not The One" - Daniel Bedingfield (huge one for me)
"Back At One" - Brian McKnight (plus probably 10+ other B. McKnight songs...)
"Goodbye" - Jagged Edge
"Differences" - Ginuwine
"The One I Gave My Heart To" - Aaliyah
"Rock The Boat" - Aaliyah (only because that was her last video and inevitably, the video that took her life)
"Miss You" - Aaliyah (the video killed it for me)
"Crazy Over You" - 112
"Your Letter" - 112
"Moving On" - Toya
"Drowning" - Backstreet Boys (

ewww, I know, but it makes me all emotional)
"I'll Be Missing You" - Puffy, Faith Evans, and 112
"Why Do I Feel So Sad" - Alicia Keys
"Inevitable" - Shakira
"Fall" - Michelle Branch
"Have You Ever" - Brandy
"Love Is Blind" - Eve
"One Sweet Day" - Mariah Carey and Boyz II Men
"Can't Believe" - Faith Evans and Carl Thomas
There are so many more, I can't even think.