Also not really opethy, but cool doom/death are November's Doom...not really familiar with there old stuff but i heard "Novella Resovoir" from this year and loved it.
Oh well, finally someone mentioned When Day Descends. The album "Transcend" is the most Opethian thing I've ever heard, pretty much like an instrumental version of Opeth, with loads of acoustic stuff. Even the album artwork looks like Opeth. Nice band.
Hmmm... I didn't know October Tide, but judging by what I've heard from their myspace they seem to be quite a good recommendation as well