Novembers Doom is somewhat similar, more death/doom though.. Paul has an excellent voice. You'll also want to check out Paul's side project, Subterranean Masquerade, awesome psychedelic prog with jazz influences and death metal vocals.
Then there's Dark Suns. Their album Swanlike is more like Opeth, while Existence reminds me a lot of newer Katatonia. Brilliant, anyways.
Uh, people who like Opeth usually like Agalloch too, even though the bands aren't similar in any way.
Check out Atheist and Cynic for some slightly proggy oldschool death metal.
Also, Katatonia's Brave Murder day has Mike doing vocals.
And finally, check out everything Dan Swanö has done. Mike does vocals on Edge of Sanity's Crimson, and everything else by EoS is fantastic. Nightingale isn't anything like Opeth, but an awesome prog/hard rock band anyways.
And then also Steel. Mike plays guitar. Nothing like Opeth, power metal that doesn't take itself seriously.
Phew, that's a few recommendations that should keep you busy for a while.