Israel "Iz" Kamakawiwo'ole's "Over the Rainbow / What a Wonderful World"
My wife and I went to see "50 First Dates" last night. They use the song in the closing credits. I first heard the tune used in the old eToys commercials in the late 90's - the dad watching his son try to catch fireflies, then shopping for a gift online and seeing a bug cage on eToys - really moving and memorable music.
The song has been used in "Meet Joe Black", "Finding Forrester" and "ER" as well.
I tried to pick it up using iTunes, but for some reason all of Iz's stuff has been taken down. So I googled "another source". You can hear the song here