"Somewhere Over the Rainbow" by Israel Kamakawiso'ole:
The story behind this is that I loved this version of the song - it had been used on some commercial years ago. Then...this was playing during the scene on ER when Mark Greene died. The episode aired a few months after my ex-husband left - I sat and sobbed.
I looked up the song and bought the CD - I love it and now have it on my iPhone. Just listened to it, and even almost 8 years later, and being happily re-married for almost 5, it still brings me to tears.