The post-metal related genres are among my favorites.
I strongly second the Altar of Plagues recommendation. They are similar to Fen, but with a bit more post-rock influence than them, if I remember correctly. Their debut is absolutely fantastic.
Also seconding Fen.
I only have the Temporary Antennae
from Caïna, and I haven't noticed too
many similarities between them and Fen/Altar of Plagues. But you already know about them.
Since you already know Cult of Luna, and undoubtedly Isis (you DO
know Isis, right?), Callisto should be an obvious choice. I've just started listening to them recently, in fact, but they are great.
Neurosis seem to get put in this crowd fairly often, being that they really helped kickstart it, but I don't think they are actually a part of the same genre (I'm not an expert though, so correct me if I'm wrong.); regardless, you will probably like The Eye of Every Storm
Amesoeurs debut/departure album fits the style at times, though it also dabbles with a more black metal approach, as well as shoegaze. Lots of different styles within the album, so I recommend it, because at least a couple of them are bound to fit your taste.
While not exactly metal (though there is some metal influence), Irepress may work for you as well. They pull in a lot of different styles, kind of a smorgasbord of instrumental post-rock/prog/metal and who knows what. I don't mean that they jump around from genre to genre though, I mean that they blend the styles to create their own quite unique sound. Both of their releases are quite good, but I prefer their debut, Samus Octology
. They may or may not fit be what you're looking for, though.
Seconding Agalloch's Ashes Against the Grain
. A marvelous album (One might call me an Agalloch fanboy, though, so take it as you will.), though certainly not so entrenched with the post-rock sound as some of the other bands listed here, like Isis and Cult of Luna.
I'm going to assume you've heard Isis already, but if you haven't, check out all of their full-lengths, particularly those after Celestial
. Isis are easily my favorite band in the genre. (I'm also an Isis fanboy, though
Battle of Mice is an interesting band in the genre. I found out about them when their debut came out. They've got a female frontman (err, front-woman), so that's a nice change of things. Some of the songs work really well, others, not so much. Regardless, you may like them, so give their debut a shot. They sound pretty different from the other bands listed.
Caldera is solid an stoner/doom/post-metal band. I gave their album a few spins. Noktorn's review is pretty spot-on. They're good, and fresh, but not really amazing, in my opinion. Certainly worth listening to, at the very least.
Giant Squid are a band that I just recently checked out. Their debut is pretty interesting. I can't tell you much about it, because I didn't give it a thorough listen, but I do remember it being loaded with atmosphere, and I think it had some cellos and whatnot, if I'm not mistaken.
I heard good things about Intronaut and downloaded some of their stuff, but I've yet to check it out yet.
Pelican's The Fire in Our Throats Will Beckon the Thaw
is also worth noting, as Ilwhyan already pointed out. They always seemed to have a very grand soundscape to me. I don't really care for their newest album.
I guess that's about it.