My favorite suggestions so far are "Bohemian Rhapsody" by Queen, "Somewhere Over the Rainbow" by Israel Kamakawiwo'ole, and "Don't You Forget About Me" from the Breakfast Club Soundtrack.
Other ones I'd like:
Flaming Lips - "Do You Realize"
http://www.youtube.com/watch?v=5zYOKFjpm9s
The Beatles - "Because", "Eleanor Rigby", "Across the Universe", or "Hello Goodbye"
Frank Sinatra - "That's Life"
And, because it fits the situation,
Franz Ferdinand - "Take Me Out"
http://www.youtube.com/watch?v=QM4dxI0mO1k