well,
I am huge fan of In the Woods... and all their albums are great.
The closest link they share with Opeth is long songs and the difference between every album.
HEart of the Ages is black metal with strong doom influence(Ulver a bit) and the singing is awesome. Omnio is absolute masterpiece, EVERY Opeth fan should have a copy. Please don't let me explain why, just GET a copy and hear for yourself.
Strange in Stereo and 3 Times 7 on a Pilgrimage are also great but the first two are better examples for the 'beginners'. And Return to the Isle of Men is rerelease of ItW... demo and it's the only ItW... thing I don't like, but everything they recorded is 100% great(sounds familiar?).
Blazing Eternity's Times and Unknown Waters is released on Prophecy Productions, ans as all of you should already know by now, every Prophecy band is great. So is Autumnblaze and Drawn. So is Tenhi and Naervaer. And Green Carnation and Empyrium also etc, etc.
Cheerz