As I have said countless times ...... opeth > *
Nothing else sounds like opeth.
Porcupine Tree is certainly worth listening to (good recomendation there CorporateRecreation)
Ayreon plays progressive metal/rock opera that is at a similar "level" as opeth. (Check out Lucassen's Star One as well)
As someone mentioned before, Iced Earth has their share of Epic songs, but they really aren't anything like Opeth. (still awesome though)
I really love Nightwish as their music has a lot of beauty and emotion, but it's not nearly anywhere close to the same level of technicallity as Opeth
Edge of Sanity has some really awesome and epic stuff. They do not have the same style as Opeth, however, it's one of the closest matches I can think of. Also, Dan Swano is a god.
Bloodbath is a heck of a lot heavier, however, Mikael Ackerfeldt from Opeth performed on their first 2 releases. You might want to listen to their material.
Katatonia released a few CDs with Mikael Ackerfeldt as well, so you may want to listen to some of their music.
EDIT: Some of the older Amorphis Material from the 90s was pretty interesting/good/different/out there ...... might be worth listening too as well, though their last few releases have been garbage IMO