A few comments on some bands mentioned:
Agalloch. Novembre:
Both have similar feels to opeth, and some similarities, but there are also some fundamental differences, and for these two bands they tend to have similar themes throughout their songs and tend to work on variations of riffs, and mixing of ideas throughout songs. Opeth of course arent like this, they use a new riff for every occasion, or use an exact replica of a previous riff. Makes for quite different music, to get the most from both bands you'd have to listen to them in quite different ways. To me, agalloch and novembre also both sound more standard minor scale than opeth (who use a lot of diminished chords, and disonance, and jazzy chords, and such). And these two bands also use more effects and such. Id say while there are similarities in song structure and some feels and riffs, that overall the fundamental differences in how they express themselves through music is quite large.
Maudlin of the Well:
Completely different musical attitudes. Opeth become more 'riffy' by the day, maudlin's next album is attempting to compeltely remove the idea of the riff. Maudlin have the variety in riffs and influences that opeth have, but explore ideas for far longer and change things up and whatever. Again, similar in some ways, but the fundamental ways they create music is extremely different.
Katatonia:
Most doesnt have similarities, but the BMD period does have similarities to what opeth do. Its different but a song such as Rainroom would not be out of place on mayh (assuming it had the same production) i would think. Its about as close to opeth as ive heard. As always there are differences, and recent katatonia bares no resemblence at all. The main difference with katatonia and opeth is that opeth create intricate and interesting riffs, katatonia have always worked more on droning riffs, distorted stuff tends to be stuff like having a chord and just strumming it 8 times on the beat. When they get notey on BMD its more in the orchid/morningrise style, though a bit less twin guitary (tends to be just straight harmonies of a line).
Thales and Farmakon both have similarities but again have their differences. Both are pretty close to opeth though, at least they are starting from an opeth position, and have developed their own styles from there.. and as such some of the fundamentals are the same (especially the whole idea of changing riff completely, and the quick change between acoustic/distorted, and song length, and song structure, and variety of sounds.. etc).
I havent found any band quite like opeth, but there are plenty that do elements of opeth's sound, and each has their own uniqueness to them as well. There are differences with the above bands for sure, but i would say if you like opeth then there is plenty chance you'll really get into some of the above bands.