I find that listening to these types of songs when I’m down tend to help me put my problems into perspective with the grand, incomprehensible orchestration that is the universe. Two that come to mind immediately are:
Don Mclean - Tapestry
Kansas - Dust in the Wind
What others make you feel small?
Motorhead - Ace of Spades
I’m here for you, buddy.
Tool - Parabola (a lyric snippit can be found in my sig)
Deteriorata-- National Lampoon
You are a fluke of the universe
You have no right to be here
And wherever you are
The universe is laughing behind your back
Whenever I make a CD full of heavy, serious songs, I like to throw in Pharcyde’s “Your Mom is so Fat” to lend a little perspective to the whole thing.
Monty Python: “Bright Side of Life”
“Life’s a piece of shit,
When you look at it.
Life’s a laugh and death’s a joke. It’s true.
You’ll see it’s all a show.
Keep 'em laughing as you go.
Just remember that the last laugh is on you.
And…
Always look on the bright side of life…”
Listen to these, cry your eyes out, and get it over with. 
Good Time Charley:
http://www.mareesalbumlyrics.com/Good_Time_Charlies_Got_The_Blues.html
Rainy Night in Georgia:
http://www.lyricsdepot.com/brook-benton/rainy-night-in-georgia.html
Alone Again:
http://www.lyricsxp.com/lyrics/a/alone_again_naturally_gilbert_o_sullivan.html
Misty:
http://www.lyricsdepot.com/lloyd-price/misty.html
We’re All Alone:
http://www.lyricsdepot.com/rita-coolidge/were-all-alone.html
“Box of Rain” - Dead
Jack and Diane, John Cougar Mellencamp
Listen to the blues, kid. I guarantee it’ll put your problems into perspective with the grand, incomprehensible orchestration that is the universe. Migt even cheer your ass up.
One for the Aussies:
Jimmy Barnes - “Ain’t no second prize” Not especially uplifting or deep and meaningful. Motivational more than anything and a damn good tune. Barnesey at his best. (IMO)
“The Hokey Pokey”
" It All Comes Out in the Wash " by self nails the futility of day to day life.
All I want from life
Is a car that I can drive
Home from work - late at night
From my 9 to 5
And If I don’t survive
I guess all I want from life
Is to know that I tried before I died
Beth Nielsen Chapman - Sand and Water
Oddly enough, a surprisingly truthful bit of a Wierd Al song.
Everything you know is wrong.
Black is white, up is down and short is long.
And everything you thought which is so important -
Doesn’t matter.
“Battle of New Orleans” by Johnny Horton.
“Just remember when you’re feeling very small and insecure
How incredibly unlikely is your birth
And pray that there’s intelligent life somewhere out in space
'Cause there’s bugger all down here on Earth”
Damn it, it’s “Galaxy Song.”
At least I was close.
“Alive in the World” by Jackson Browne
Pink Floyd’s Time, from Dark Side of the Moon, lyrics by Roger Waters:
Emphasis mine. One of the most chilling ways to think about a wasted day. (Like today, for me.)