Since I didn't leave comments on these, I'll say something -
Eyehategod are one of the key sludge metal bands, maybe THE key one besides Melvins (who someone else posted), though bands like Acid Bath (also above), Down, Neurosis, Buzzov*en, and Dystopia are also significant. EHG are my favorite for just communicating pure misery. They wanted to sound like "if Black Sabbath was a punk band," though there's also a strong southern blues element.
Behemoth are an amazing semi-mainstream death metal band comparable to bands like Gojira. If you like their style but want something more underground, I'd also recommend Archgoat and Beherit. This isn't the best Behemoth video, but I wanted to promote House of Strombo, George Strombo's new music/talk show (he's a great journalist).
Napalm Death are actually a fascinating band that gets underrated for how thoughtful they were. They had their roots in the Crass-centered anarcho-punk scene and were very influenced by early crust punk, but they tied in major Celtic Frost and Slayer influences to their sound, spawning grindcore. They're connected to lots of other major bands, like Carcass and Godflesh/Jesu. As time went on, they became more and more death metal, but always with a noisy avant-garde edge.
Martyrdöd are a really great band that fuses melodic death metal and crust punk, with a little black metal. They have their roots in the band Skitsystem.
Dawn Ray'd are a solid black metal band with references to peasant revolts, paganism, and revolutionary themes. Sorta like the Rage Against the Machine of black metal. They're notable contemporaries of Panopticon.Post too long. Click here to view the full text.