Sunn O)))- extremely unusual band, might take a while to get into them. Extremely bass-heavy, very slow progressing songs, average song length is like 15 minutes (excluding outliers). A lot of instrumental stuff, pretty minimalist. Dark ambient/atmospheric, drone metal, and all that jazz.
Khanate- drone, again, and the guy's vocals are just...oh god. Definitely a band that would take some time to get into if you're not already into that kind of stuff.
Earth- slightly less unheard of, but not that common of a name, still. Instrumental stuff, fairly refined, but long and interesting.
The Church- where to start. They had one really popular album ("Starfish", way back in the 80s) and their latest one has been compared to "Wish You Were Here" by Pink Floyd. Generally space-rocky, but they do some more conventional stuff. Some of their earlier stuff is very 80s-rock, but still great. Some of their stuff is just plain amazing to listen to. I'd start with either "After Everything Now This" (spacey, an "easier" listen, you could say), "Hologram of Baal" (same), "Priest=Aura" (a bit...darker, a kind of unusual album, but easily their magnum opus, I say) or their newest one, "Untitled #23" (haven't listened to it much, but some of the comparisons to "Wish You Were Here" aren't far off, I think).
Sigur Ros- another name that's large in the right circles, but not very big elsewhere. Spacey, emotional, cathartic stuff. Definite progression of styles across albums. They tend to do "big" sounds, except on their latest album ("Med Sud i Eyrum Vid Spilum Endalaust", which has eths and not d's in it, and a lot of accent marks), which is a soft, minimalist album. Their first and least popular album, "Von", is, simply put, an ambient/experimental album to put everyone else to shame. "Agaetis Byrjun", I honestly believe, trumps "Dark Side of the Moon" and "The Wall" with ease, which I hold as two of the greatest albums ever recorded (though that is different from "favorite albums"). The second song on it, "Svefn-g-Englar", puts at least 95% of all other music to absolute pitiful shame. You can download it off their website if you want, along with some other stuff from their various albums (it's sigur-ros.co.uk).
Motograter- Heard of Five Finger Death Punch? This is the vocalist's (Ivan Moody) old band. Makes Linkin Park look like a pansy band. Energetic, aggressive, abusive to most people's ears (not because it's shit, though), and might scare the little 'uns. But if you want heavy/thrasher metal, this is the shit for you.
Filter- not sure how popular they are, so posting this one anyways. Fairly conventional pop/metal-ish. Not much to say otherwise. Good stuff, though.
Neurosis- see above. Avant-garde metal band. They're...odd, but interesting.
Isis- also, not sure of popularity, but I know it's not that huge. Another avant-garde metal band, whom I prefer to Neurosis. Long songs (almost all of them, and I mean with like three exceptions (barring segue/non-song tracks) are over 5 minutes, most over 6, some over 9/10). "Celestial" is extremely rough, very aggressive, and also extremely interesting. "Oceanic" is one of the heaviest/head-bangingest albums I've ever heard. Don't have "Panopticon" (sadly). "In The Absence of Truth" is a good album, not as good as the others, but has its awesome moments (specifically, "Holy Tears" and "Not In Rivers, but in Drops"). Don't have "Wavering Radiant".
Morphine- soft/"low" rock. It's a jazz/blues/rock mix band, and the vocalist has a deep, kind of gravely, very soothing/soft voice. "The Night" is the only album I've been able to get my hands on by them, and it's just flat-out epic.