hmmm i have far too many, have 60gb ipod photo and it's about half full lol!
some of the ones that no one has mentioned yet:
Ani Difranco
Beth Hart
Depeche mode
Donna Summer (hehe, couldn't help it!)
the doors
Janis Joplin
Justin Timerlake
Billie Holiday
madeleine peyroux (sounds kinda like billie holiday)
jane monheit (beautiful voice... love her!)
Michael Buble
Tori Amos
Rufus wainwright
Elliott Smith
Moulin Rouge soundtrack
RENT soundtrack (both original broadway & movie)
Wicked soundtrack
AVE Q soundtrack
Fiona Apple
Gorillaz
Jack Johnson
John Mayer
The lovemakers
My chemical romance
nine inch nails
Panic! at the disco
PJ harvey
The Raconteurs
Regina Spektor
Radiohead
Sarah brightman
Tegan & Sara
Violent Femmes
there are tons more, but I'll stop now, hehe
ps: postal service, Bjork(!), Scissor sisters, and the Killers are awesome!!!