i look forward to checking out all the links in this thread over the next few days. my stoner buddy has been trying to get me into Opeth for years. if you like them he also recommends Arcturus, Amorphis, and Borknagar - i know nothing about any of these bands.
Borknagar is AWESOME, although might be a little on the extreme side for people here. They are SO melodic, though. And they incorporate this really interesting kind of "folk vocal" over the black metal main vocals. Wait til you get to the break at 1:40. Sit through the blast beats, you'll be okay trust me.
Amorphis I LOVE because they incorporate a metal element into a more traditional rock song format.
This might be one of my favorite songs, ever, period with some of my favorite melodies, EVER.
Check this out: