Here I go, this is all metal.
Amon Amarth - Death Metal, but really awesome and slower than other Death Metal bands out there
Anthrax - Thrash Metal, one of the big four, really awesome
Atheist - Technical Death Metal I guess, but lighter than most Tech Death bands out there
Blotted Science - Progressive Metal, and totally instrumental. Really great songs.
Cryptopsy - Technical Death Metal, but completely different than Atheist. Very faster and harder stuff.
Death - The founders of Death Metal, they have their similarities with Atheist, albeit they have a harder sound, nowhere near Cryptopsy though.
Dream Theater - Progressive Metal, very light stuff... though it's worth listening to.
Electric Wizard - Stoner Metal, the only band I have that plays stoner metal, but it lets me know that I should get into more Stoner bands... very heavy and slow stuff.
Fear Factory - Industrial/Thrash/Groove/Death Metal. Really awesome. That is all.
Iron Maiden - Really, one of the most famous Metal bands out there. They pioneered the New Wave of British Heavy Metal movement... they are very light... but no one can call themselves a metalhead until they have at least one of Iron Maiden's albums.
Jonnah - A very new band, that plays a sound that now gets categorized as Djent. Think Meshuggah.
Kreator - I've heard them get called Teutonic Thrash Metal, and I'm not sure what that means... but what I do know that they have an awesome, heavy, fast sound. They also were a major influence on Death Metal.
Manowar - Heavy/Power Metal. Very epic sound, their lyrics are especially great.
Meshuggah - One of the most innovative bands out there, they single-handedly invented a genre, Djent. It's a very interesting sound, would definitely recommend it if you're bored of other kinds of Metal
Metallica - Thrash Metal, again one of the big four, you pretty much love Metallica or hate them.
Morbid Angel - Death Metal, a heavy sound. Can't really comment any further than that they're great.
Napalm Death - The fathers of Grindcore. Grindcore is a very hard, heavy and rough sound, that not everyone can listen to.
Nile - Technical Death Metal... these guys really are good. They have great technical prowess and are really worth checking out.
Origin - Wikipedia says that they are "Brutal Technical Death Metal", haven't heard that before. But I'd say that describes them perfectly. Very brutal... just nonstop action.
Periphery - Again a new Djent band, but lighter than the other ones.
Rammstein - Neue Deutsche Härte, or rather, Industrial Metal. My favorite band on this list, I'd really recommend them to anyone, even if they're not a fan of metal.
RXYZYRX - Djent, that's all there is to it.
Sensory Amusia - The heaviest of Djent bands I've heard so far.
Sepultura - Thrash/Alternative/Groove/Death Metal. I don't really know. Definitely not my favorite band, but not my least favorite either.
Slayer - My favorite of the four big Thrash Metal bands. They have a very fast, thrash sound. Definitely recommended.
Strapping Young Lad - Experimental/Extreme/Industrial/Heavy/Progressive Metal. Another one of Devin Townsend's projects. This one is my favorite.
System of a Down - Alternative Metal, I guess. They are very fast, they have very interesting songs and are a fun listen.
Vader - Death Metal. Very rough, heavy sound.
Venom - Thrash/Speed Metal. Again, not the most favorite of bands, but I like them.
Vildjharta - Again Djent, but one of the best ones in it.