Forget all the bullshit that people here have written: There is NO similarity to Drudkh or Empyrium (are you people fucking deaf or stupid, which one?) and defnitely not Borknagar (hell, are you fucking tone deaf? See your fucking doctor!).
The closest that you will get to Agalloch really is their sister band, Sculptured and even then Sculptured is less post-rockish than Agalloch is, and much more Maiden influenced, and even has brass. Still, the inclusion of Don Anderson, Haughm and JWW in the band is enough to make it sound Agalloch-ish.
Tom might be able to suggest some post-rock bands as that's not really my forte.
To find similarities or influences to Agalloch, aside from Ulver's Bergtatt (which they have cited as an influence in interviews), you are best off exploring their non-metal aspects: investigate bands like "Godspeed! You Black Emperor", investigate indie film (probably their biggest influence overall).
I generally find that despite being a simpler album for Agalloch, Ashes Against The Grain is probably the most complex one to get into. Thrash out The Mantle first, then go onto Pale Folklore, then into Ashes Against the Grain.
As far as Drudkh goes though, I suggest holding off first until you settle with Agalloch. Not because they're similar, but because Drudkh's journey is on a totally different path, and also further into the core of Black Metal's cultural ideology. With Drudkh, you must immerse yourself and your approach - it is not music created for the sake of sounding good.