Cynic kick ass, and both Cynic and Atheist have been mentioned already.
Isn't a good bit of Black Metal Progressive? Even Emperor can be placed in the category. And some people call Wintersun progressive, but they are pretty much accessible...more Power than Prog.
Green Carnation and Orphaned Land are good bands as well.
Pavor (free form styled technical death metal)
Sworn (like a heavier version of Opeth. progressive doomdeath, I guess)
Coram Lethe (kinda like Cynic, but IMO they are better)
Scald (the Irish one, not the Russian doom band. progressive extreme metal)
Ordog (very strange progressive raw black metal)
The Elysian Fields (progressive blackened death)
Devin Townsend, Meshuggah, Opeth, Into Eternity, Biomechanical, Mastodon, Wintersun, Communic, Death, Sikth, Akercocke, Dodheimsgard, Disillusion, Cynic, Textures, Unexpect, Atheist, Enslaved & well that pretty much sums up my thoughts so far. Good luck! =) I might have some use for this thread as well.
What about the album "Buried in Oblivion" by Into Eternity?
I like this album very much...
Also how about those greek brothers Wastefall and especially their splendid albums "Soulrain21" and "Self Exile"...Not so exterme but certainly heavy and progressive!
Have fun dude!