Stevie Nicks
Jimi Hendryx
Billy Holiday
Jackie Wilson
Marvin Gaye
Gordon Lightfoot
Frank Sinatra
Tony Bennett
Jerry Vale
Marty Robbins
Which one? Bon Scott (pre-Back in Black) or Brian Johnson (since)?
Nat King Cole
Leadbelly
No one has mentioned Joe Walsh yet?
David Lee Roth and Sammy Hagar have pretty distinctive voices, too. So does Steven Tyler. And add former J. Geils’er Peter Wolf.
Luther Vandross
Prince
Richard Pryor
Barry White
Lou Rawls
Gilbert Gottfried
[Gilbert Gottfried]i’m altering the deal. Pray I don’t alter it any further[/Gilbert Gottfried]
Tom Petty
Gord Downie
Win Butler and Regine Chassagne (only together, strangely enough)
Mama Cass, anyone???
Morgan Freeman. Well, except the dude from the Allstate Insurance commercials seems to be a ‘wannabe Morgan’.
Keep making remarks like that and see how long it takes until Jack Bauer is shoving ball point pens into your knee cap! Blasphemer! 
To take this down a heavier path:
King Diamond
Phil Anselmo (Especially during the Cowboys/Vulgar period)
Chuck Billy (Testament)
Dave Mustaine (Megadeth)
Don’t know his name, but the singer for Exodus
Axl Rose
Rob Zombie
A pair of Johns: Fogerty and Mellencamp.
Jimmy Somerville (Formerly of Bronski Beat and the Communards)
Nico
Chuck D from Public Enemy
Mike Mills and Michael Stipe from R.E.M.
G.G. Allin
Alison Krauss
Garrison Keillor
Tom and Ray Magliozzi
Sean Connery