It’s playing right now in my CD player: The Hustle.
Sweet Home Alabama
No, wait…that’s only when I’m feeling blue.
Jungle Brothers… Because I Got It Like That
Kein Bestandteil Sein/To Be No Part Of It - Einstuerzende Neubauten
Weird, I’m sure, but it always makes me feel better. Memories and all that.
Therion’s “Wine Of Aluqah”. There’s no way you can stay down after listening to the second half of that.
Should I stay or should I go ~ the Clash
Call me ~ Blondie
Material Girl ~ Madonna
Walk like an Egyptian ~ Bangles
Bleah!
Oooo…that’s a good one.
In The Hands Of Death by Rob Zombie with Alice Cooper
Jessica is my personal favorite, never-fail smiley song 
You asked for it - in no particular order, after a brief perusal of my collection:
Save Ferris - Nobody But Me
Ramones - I Don’t Want to Grow Up
Sublime - 40oz. to Freedom, Same in the End
Bad Religion - I Want to Conquer the World
NOFX - August 8th
Tom Waits - Blue Skies
Bob Marley - Three Little Birds
Led Zeppelin - D’yer Mak’er
Van Morrison - Days Like This
Pink Floyd - Comfortably Numb
George Jones - Finally Friday
Willie Nelson - On the Road Again
Jimmy Buffet - Son of a Sailor, Margaritaville
There’s so many more, including most of the ones already mentioned by others, but I think I’ll spare you and leave it at that. Really, I think any music I like picks me up in some way when I’m feeling down - even depressing music feels nice, even if I’m already depressed enough. It’s always better than not listening to music, at any rate. I suppose it’s just comforting.
I was gonna say…Fortyoztofreedom, Same in the End, Comfortably Numb, Nobody But Me, and Margaritaville are all downers.
August 8th is about being glad that Jerry Garcia is dead.
Three Little Birds definitely has a place here though :D.
“Escape” by /V)etallic/): “Life’s for my own to live my own way” amongst other good bits… <side note> This is the one song Hetfield says he regrets writing. </side note>
The theme from Top Gun
The video for “Rainbow in the Dark” by Dio (or just the song if i’m not at my comp)
Got a couple myself, but the big one is ‘King of Spain’ by Moxy Fruvous. Darn song gets me dancing in my chair even if I’m having the worst day ever. 
Rome wasn’t built in a day by Morcheeba
you and me, were meant to be
walking free, in harmony
one fine day, I’ll fly away
don’t you know that Rome wasn’t buiilt in a day
Its “The Drive Home” by Chicane for me.
In fact the whole of that album never fails to cheer me up…
A lot of Bob [that’s Bob Marley to the rest of you :D]'s stuff works for me, too.
Also: the theme from Shaft.
That’s an awesome song. I also really like “Fear & Love” by Morcheeba.
Yeah, I have a warped sense of what makes me feel better. Songs that are too happy tend to just piss me off, rather than put a smile on my face, if I’m not feeling well.
And besides, how can a song talking about crying hippies not make you feel better (ok, unless you’re a big Grateful Dead fan)? 
Speaking of which, there’s a Grateful Dead song that’s perfect for that list too… I just can’t remember the title of the song I’m thinking of right now!
Here Comes The Sun
Works every time.
Beastie Boys always bring a smile. Especially “Girls.”