Wow! I'm shocked, and glad, to see so many other metalheads out there who are also Prog fans! And I thought my friends and I were the only ones. Lol.
I guess Necksnappa didn't stop to realize that so many people listed King Crimson....because King Crimson is amazing!!! So much of their old stuff was quite heavy and metal for it's time, Court Of The Crimson King, Larks Tongue in Aspic, and Red, for example, just about killed anything else out at the time that dared to think of themselves as heavy. And their old music is just as intense today. Also, I see not many of you if any listed any newer crimson albums. You owe it to yourself to check out The Power To Believe, and The Construkction of Light, their newest studio releases. (TCOL from 2000, TPTB from 2003.) Very different from their older, proggier stuff, but just as intense, complex and weird as ever, but a whole hell of a lot heavier! Some really heavy stuff there, especially on Power To Believe, and anything live.
Some other prog favs beyond crimson:
The Flower Kings
Spock's Beard
Marillion
Under The Sun
Cairo
Dream Theater
Symphony X
Transatlantic
Porcupine Tree
ELP
Genesis
Yes
And tons more I shall have to raid my collection to name.