All right......there are too many to list, but here goes a list of my favs anyways (I listed their respective bands, just in case some aren't familiar with the singer's actual name):

Anthony Kiedis - The Red Hot Chili Peppers
Eddie Vedder - Pearl Jam
James Hetfield - Metallica
Kurt Cobain - Nirvana
Gord Downie - The Tragically Hip
James Maynard Keenan - Tool, A Perfect Circle
Billy Corgan - Smashing Pumpkins
Chris Cornell - Soundgarden, Audioslave
Layne Staley - Alice In Chains
Jerry Cantrell - Alice In Chains
Edwin - I Mother Earth
Scott Weiland - Stone Temple Pilots, Velvet Revolver
Pete Loeffler - Chevelle
Ed Kowalczyk - Live
Chad Kreuger - Nickleback
Rob Zombie - White Zombie, Rob Zombie

Recognized as some of the greatest, but not necessarily my favorites:
Robert Plant - Led Zepplin
Johnny Cash
Elvis Presley
Steve Tyler - Aerosmith
Axle Rose - Guns 'n' Roses
Ozzie Osbourne - Black Sabbath, Ozzie
Mick Jagger - The Rolling Stones

like I said, there's far too many to list off the top of my head, but that was a few.