Ke$ha, Vanessa Carlton, M.I.A., KT Tunstall, Michelle Branch.
Oh, and Jessie J And Lana

Too many to list down

Lady Gaga
Kanye West
Drake
Ke$ha
Katy Perry
Skrillex
The Weeknd
Owl City
Marina and the Diamonds
Madonna (Basically)
Bjork (Basically)
Lana Del Rey
Florence + the Machine

Katy's, Marina's & Britney's... I used to have Rihanna's but I lost my A Girl Like Me.

P!nk
No Doubt
Britney Spears
Gwen Stefani
Rihanna
Christina Aguilera
Lady Gaga
Nicki Minaj
Nelly Furtado
Marina & The Diamonds
Ellie Goulding
Ke$ha
Adele
Florence & The Machine
Calvin Harris
Katy Perry
Jessie J
Beyoncé

Destiny's Child (yeah, including Love: Destiny, 8 Days, This Is The Remix etc.)
Queen Bee.

I'm running to get M.I.A.'s, Missy's and Janet's as well.

Carrie Underwood
Lady Gaga
Black Eyed Peas
Katy Perry
Kelly Clarkson
Britney Spears
P!nk
Coldplay
Usher
Ke$ha (if it counts since she only has 1 album)
Eminem
Drake
Lil Wayne
Jason Aldean
Luke Bryan
Brad Paisely
Beyonce
Adele
Maroon 5
Lady Antebellum
Bon Jovi

And a few others. I have way too many

Excluding artists with only one album:

Basement Jaxx
Bat For Lashes
Bloc Party
Evanescence
Feeder
Florence + The Machine
Franz Ferdinand
Garbage
Hard-Fi
James Blunt
Joss Stone
Justice
Kasabian
Kelis
Lily Allen
Little Dragon
M.I.A.
Marina & The Diamonds
Maroon 5
Noisettes
P!nk
Paloma Faith
R.E.M.
Red Hot Chili Peppers
Santigold
Shania Twain
Snow Patrol
Texas
The Ting Tings

Gaga
Kesha
Nicki Minaj ( I dont' have The Boys yet though)

I'm not sure if she has much more than studio albums so;
Marina & The Diamonds

And debut artists
Rita Ora
Natalia Kills
Lana Del Rey
Gabrielle