I Need Some Recomendations...

Carta's mentioned Isis and Cult of Luna, which are probably safe bets since Pelican is basically Isis without vocals.

Neither of those two would exist without the trailblazing of Neurosis, so add them to the list.

Also, I don't think anyone mentioned Zero Hour - very heavy with progressive influences.

Amorphis perhaps? I'm willing to bet the OP is already familiar with most of these suggestions....
 
would Radiohead be concidered progressive?

Not really. The only time they seem 'progressive', are bits on OK Computer. Sure, they've progressed from album to album, but the music itself isn't what you'd call progressive. But I do know they have progressive influences.

Mastodon?

I've got to agree with you there. I hear the word progressive brandished about everywhere whenever I look into Mastodon, but I've listened to two of their albums (Leviathan and Blood Mountain) and found both of them to not only be rather un-progressive, but pretty boring as well.
 
If you had to brand them then the easiest way to describe Radiohead these days is experimental.
 
In just about every way possible since Kid A. Man this forum is really something.
 
Radiohead is a band that is always experimenting with different styles, different instruments, different prouction techniques, different ways to manipulate things in unconventional ways to create sounds. That is Radiohead like them or not.
 
Truly some genius logic you've got there(sarcasm.) It's an unconventional in the way they make sounds. Just because a band is not mainstream doesn't mean they're trying new, and innovational ways in going about making music. Besides Radiohead is pretty darn mainstream, which is the term coined for bands fairly well known.