In no order...
Black Sabbath
Iron Maiden
Slayer
Megadeth
God I can't bear to leave out Judas Priest, Wintersun, Amon Amarth, Alestorm, (early) Metallica, Arsis, Enfsiferum, Dethklok, Overkill, Dio, Testament, Helloween, Finntroll, Ozzy, or Pantera.
I hate list threads because I listen to too much metal... averaging 7-8 albums a day in the summer. I can't choose between Alestorm and Overkill. It's too hard.
Also, Im pretty sure nu metal origanated as a crossover of hip hop and metal. So if you can call punk rock rock, you can call nu metal metal. Although all nu metal has contributed to the genre is simplification and "underground popularity". I swear the some people at my school thought that Disturbed and Avenged Sevenfold were underground, and that they were really really big metalheads for listening to them.
As for metalcore http://www.youtube.com/watch?v=bjg8mmVUuxw. That is all.
Also, is grunge a derivative form of metal??? It does fit fairly well with some light doom...
I enjoy procrastinating on these forums.
Also SOAD (/Scars or Serj) is awesome, but not metal IMO.