Oooooohhh, so many awesome bands ...
I'm gonna second Canai's recommendation of Ayreon. Ayreon is fvcking awesomely awesome. I'd even venture to say that they are awesome at the Opeth level! You can't go wrong with any of the albums, but if you only were to sample 1, be sure to give "The Human Equation" a listen. Mikael Akerfeldt is one of the guest vocalists on that album.
Also, it's worth checking out Blackfield an No Man (Steve Wilson's other bands) They aren't really "heavy", but the music still rocks.
I've only heard a couple of tracks, but the new testament sounds a lot like testament from the early 90s. It's good thrashy stuff. It doesn't have the "deathy" sound like they tried to do later in the 90s. It's not among my personal favorites, but still good music!
If you want any really crazy shiat, listen to Bloodbath's first or second CD. Mikael from Opeth is in that (as well as some Katatonia as was pointed out earlier)
If you haven't heard it, check out Edge of Sanity - Crimson. Swano is a genious, and Akerfeldt's playing and vocals are great on it.
On the Doomy side of things, I can't get enough Warhorse or Electric Wizard.
Also I'm a huge Novembers Doom fan, their older stuff is pretty straight death doom, but they've really changed over the last decade. Mostly I'm recommending them since they are local, and it helps that Dan Swano mixed their CDs The Pale Haunt Departure and The Novella Reservoir.