Post-Metal:
Jesu - Their early stuff is pretty much split down the middle between post-metal and post-rock. Try Self-Titled. Epic as all hell.
Isis - These guys were also heavier in their early years. If you want a more melodic take on the post-metal stuff, try Panopticon. It's my favorite.
Boris - These guys go all over the place with their style. Literally one minute they're drone, the next they're stoner rock, the next they're post-rock... I'd suggest Pink as a starting place, as it encompasses a large portion of their sound.
Post-Rock:
Jesu - their later stuff is much more melodic, sometimes branching over into chill-out electronic or trip-hop, but it's all amazing. Seriously, my favorite band.
Red Sparowes - Instrumental post rock, with some nice drone segments. There's a lot of this kind of stuff out there, but I like these guys quite a bit.
Mogwai - One of the "big names" in post-rock. I only have one of their albums, but they're instrumental and it's all pretty high-quality stuff.
Ambient/Drone:
Eluvium - also not too familiar with everything he's done, but I've been impressed with what I've heard.
Final - The ambient electronic project of Justin Broadrick (the guy from Jesu). "3" is very good.
I feel like I'm forgetting some stuff, but I'll just post more later if I think of any.
Sadly, I've been seeing a lot of "post-metal" bands pop up lately that are really just trying to sound like Isis. It seems like even the most creative music gets watered down eventually...
Peace
Rorix