When I'm particularly feeling low, such as nowadays having been recently made redundant theres only a few artists that I can listen to that actually put me in a better mood, them being

Lady Gaga
Darin
Michael Jackson

How about everyone else?

Beyonce
Leona
Alexandra Burke
Ke$ha

The Saturdays
Britney Spears
Lady Gaga
PCD

Dancing around the living room to Madonna always cheers me up
I also agree with GaGa and MJ, great songs to bounce around to.
And, anything sung by Michael Buble, his voice never fails to make me melt .

Currently, The National.

Michael Jackson - especially Don't Stop Till You Get Enough
Lady Gaga
Rihanna
Jedward

John and Edward
Lady Gaga
Michael Jackson
ABBA
MGMT
Owl City

GaGa
Sugababes
Rihanna
Beyonce
Ke$ha
and many more...

Britney Spears
Danity Kane

And I can't believe I'm saying this, but Daisy Dares You.

MJ - Wanna Be Starting Something
MJ - Black or White, that opening guitar riff is happy sounding.
Queen - Fat Bottomed Girls

Christian Burns
Snow Patrol
The Killers
30 Seconds to Mars

Pixie Lott
N-Dubz
Jay Sean
The Killers

Girls Aloud and Lady Gaga

The Killers
MGMT
Fall Out Boy
Panic! At The Disco
My Chemical Romance
The All-American Rejects
Lostprophets
Muse
Oasis
We Are Scientists

Eminem
Ndubz
Robbie Williams
Jay Z
Chipmunk
Take that
to name just a few

At the moment it's Michael Jackson - particularly when i play Don't Stop Til You Get Enough, first thing in the morning. That song gets you going!

Oh, and I dare you to listen to this and try and feel miserable

Mine are going to be totally predictable but:

Kasabian
The Cribs
Oasis
Adam Green
Stevie Wonder
The Libertines/Babyshambles (The Blinding EP is a great mood lifter!)
ELO
The Dropkick Murphys
Roxy Music

And in a cathartic way Richard Ashcroft/The Verve and Glasvegas usually help put me in a better mood.

Beyoncé
Britney
Kelly Clarkson
Madonna
Gaga
Destiny's Child
Aqua
Basshunter lol

The Eagles
Norah Jones
Enya
Carpenters
Bruce Springsteen & The E Street Band
Coldplay - mainly live stuff

No music makes me feel happier at the moment than the upbeat, cheesy Glee cast.

Somebody To Love in particular gets me grinning like an idiot!

Mariah Carey
Alexandra Burke
Corinne Bailey Rae
Michael Jackson
Janet Jackson

Paramore
Lady GaGa

Lady Gaga
Rihanna
Beyonce
Girls Aloud
Destiny's Child
John and Edward

30 Seconds To Mars
Greenday
ELO
Snow Patrol