It's a good idea to find out how hardcore you like it.
So get some Severe Torture.
I'm not that hardcore, so some of this stuff I list may be edging away from metal in any number of directions.
List of things unmentioned, genres are my own opinion:
Workhorse Movement: Stoner Metal
Mad Trucker Gone Mad: Redneck Metal
Lollipop Lust Kill: They call themselves 'murder metal', I think that's pretty accurate.
Vaux: A friend of mine calls this 'yelling metal'
Therion: Orchestral metal. I'm told their older stuff is straight death metal, I haven't heard any though
White Lion: Hair metal.
Vomitron: They're just... wierd. I got their stuff off MP3.com before it sucked, so they may be hard to find now
Violent Work of Art: More yelling metal
Union Underground: I'd call it nu metal. You should get all the stuff that people say sucks too, form your own opinions
Stahlhammer
Grindstone
Shuvel: rap metal
Raised Fist: I think they're technically punk, sounds like metal to me though.
Pressure 4-5
Pain
Gamma Ray
Godhead
Hallow: They like to pretend they
Mr. Bungle: Very wierd stuff, probably not metal
Kill II This
Hopesfall: Melodic metal. Death metal singing with mellow music.
Edguy: Power Metal, I don't think anybody's mentioned any of that yet. I love Edguy, they're the best.
Helloween
Dog Day Sunrise
Hammerfall
Death by Stereo
Sentenced: New stuff is 'suicide metal', old stuff is death metal
Bohse Onkelz
Big Dumb Face
Andrew WK
If you liked White Zombie, try Rob Zombie, Powerman 5000 (Rob's little brother's band), and Scum of the Earth (various people from both bands).
See if you can find any metal mags, very helpfull in finding new stuff. BW+BK is pretty good, but skews a little hardcore for me.