Well, since I'm not really fond of Opeth ... I don't know a whole lot of bands that are similar to them. However, I do know a few bands that get compared to them (some I understand the comparison, some I don't) but here goes:

Agalloch - Pale Folklore, The Mantle, Ashes Against The Grain (No idea why this band is compared to Opeth, but they rule and are possibly the best band around right now, or ever)
Empyrean Sky - The Snow White Rose Of Paradise
Farmakon - A Warm Glimpse