Bands every Opeth fan should be exposed to.

The Chasm
Drudkh
Aeturnus
At the Gates(the red in the sky is ours)
Diabolical Masquerade(death's design)
Graveland
Summoning
Ulver
Borknager
Dan Swano's stuff

As far as metal goes anyway...

I'm sure ninefeetunderground could give better prog recommendations than me.
 
ok, from this point on...please don't name a band that's already been mentioned. There are only so many times you can say "agalloch" "novembers doom" and "camel"
 
This thread fails.

Essential listening other than Opeth is: Dimmu Borgir, Porcupine Tree, Aeon Spoke, Pain of Salvation, Orphaned Land, and Wintersun.