For a band that sounds similar to Opeth try In Mourning. Their album The Shrouded Divine is amazing.My favorite bands at the moment:
Anathema has sounded promising
I like tool, radiohead,
Two Anathema albums I'd recommend in particular are Alternative 4 and Judgement. Very rewarding albums. Try Riverside - Out of Myself, parts of this album remind me of Anathema.
If you like Tool and Radiohead try Porcupine Tree - Fear of a Blank Planet.