Comus. mike likes Comus. a crazy folk band from the 70s that sounds like a bunch of trolls in the forest dancing around and singing, playing flutes, violins and acoustic guitars. it's scary.
He didn't ask about what bands Opeth (or Mikael) likes, although that's always interesting information.
Morningrise's list is pretty good. Basically, if you like Opeth, you'll probably like Agalloch, Arcturus, Amorphis, Ulver, Vintersorg, etc., but it all comes down to personal preference.