This is one of few songs I really like.  It talks about a dream world where people live happily, yet is beyond our reach.  Whether this is a sad song is up to people who listen to it.  I think it would be one if you are always looking for the greener psature.

Lyrics

Somewhere over the rainbow
Way up high,
There's a land that I heard of
Once in a lullaby.

Somewhere over the rainbow
Skies are blue,
And the dreams that you dare to dream
Really do come true.

Someday I'll wish upon a star
And wake up where the clouds are far
Behind me.
Where troubles melt like lemon drops
Away above the chimney tops
That's where you'll find me.

Somewhere over the rainbow
Bluebirds fly.
Birds fly over the rainbow.
Why then, oh why can't I?

If happy little bluebirds fly
Beyond the rainbow
Why, oh why can't I?

Somewhere over the rainbow - Judy Garland

http://www.youtube.com/watch?v=WY8ip4x2e68&feature=related

Somewhere Over The Rainbow - Doris Day

http://www.youtube.com/watch?v=AxpXG2wQ-MQ&NR=1