I'll outline some basics in some sub-genres:
Death Metal:
-Cannibal Corpse
-Behemoth
-Nile
Black Metal:
-Dimmu Borgir
-Gorgoroth
-Mayhem
Folk:
-Finntroll
-Korpiklanni
-Turisas
Viking:
-Amon Amarth
-Stormwarrior
-Ensiferum
Melodic Death Metal:
-Arch Enemy
-Norther
-Children of Bodom
Thrash: (My personal Fav, I'll add some more to this one)
-Slayer
-Exodus
-Megadeth
-'83-'88 Metallica
-Annihilator
-Evile
-Anthrax
-Overkill
-Whiplash
-Paradox
-Sodom
-Destruction
-Kreator
-Darkane
-The Haunted
Power Metal:
-Gamma Ray
-Dream Evil
-Blind Guardian
Epic/Fantasy Metal:
-Rhapsody of Fire
-Manowar
-Falconer
As well as some you absolutely can't go without: Iron Maiden, Judas Priest, Pantera, Sepultura etc. Happy youtubing.
"The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents... some day the piecing together of dissociated knowledge will open up such terrifying vistas of reality, and of our frightful position therein, that we shall either go mad from the revelation or flee from the light into the peace and safety of a new Dark Age."
-H.P. Lovecraft